AGENTS.template.md

@@ -22,7 +22,7 @@ crafter-build is a C++26 modules build system. A project is described by a `proj
 
 Partitions and what they own:
 
-- `:Toolchain` resolves the compiler once per process (`GetToolchain`): clang++, clang, llvm-ar, libclang, libc++'s std module source and the default sysroots, from `CRAFTER_BUILD_LLVM_DIR`, else a toolchain installed beside crafter-build, else PATH. It also holds the version baked in at build time. `SysrootStdModule` picks the std module source for a sysroot build by the sysroot's libc++ release.
+- `:Toolchain` resolves the compiler once per process (`GetToolchain`): clang++, clang, llvm-ar, libclang, libc++'s std module source and the default sysroots, from `CRAFTER_BUILD_LLVM_DIR`, else a toolchain installed beside crafter-build, else PATH. It also holds the version baked in at build time. `SysrootStdModule` picks the std module source for a sysroot build by the sysroot's libc++ release. `MingwSysroot` settles the mingw target's `--sysroot`: the mingw-w64 prefix itself, or, for a prefix that also holds a Linux system (the host's `/usr`, anything whose `include/` has `features.h`), a view in the cache of its mingw-w64 directories, since clang would search its `include/` and `lib/` too (issue #43). `MingwViewToPrefix` turns paths into the view back into paths into the prefix for messages.
 - `:Clang` holds `Configuration`, `Build()`, `VariantId()`, `ApplyStandardArgs`, `HostTarget`, the CLI entry point `Crafter::Run`, and the lint, test and script data types.
 - `:Platform` runs commands, implements `LoadProject` (compile and load `project.cpp`), the host PCM cache, `#pragma crafter dependency`, and all OS-specific code.
 - `:Interface` and `:Implementation` model module units, their staleness checks and their compiles.
@@ -118,14 +118,14 @@ Beyond the rules:
 - There is no host-platform preprocessor split: the host is Linux. Target differences (mingw, wasm) branch on `cfg.target` strings at runtime, in the library as well as in `project.cpp` and tests.
 - A build step fails when its command exits nonzero, never because it printed. Use `RunBuildCommand` or `RunCommandChecked`; warnings go to `Progress::Diagnostic`.
 - Every output path goes through `Configuration::BinDir()`, `BuildDir()` and `PcmDir()`. Hand-rolled `path / "bin" / ...` clobbered variants before; do not add new ones.
-- Anything that changes what gets compiled or bundled belongs in `VariantId()`. It already hashes type, debug, sysroot, defines, compile flags, the wasm baseline and variant flags, uninterpreted project args and the toolchain (`Toolchain::Key()`).
+- Anything that changes what gets compiled or bundled belongs in `VariantId()`. It already hashes type, debug, sysroot, defines, compile flags, the wasm baseline and variant flags, the std module the units import (`StdPcmKey`), uninterpreted project args and the toolchain (`Toolchain::Key()`).
 
 ## Design rules
 
 These were decided deliberately. Do not undo them without the maintainer's say-so.
 
 1. **Configuration is C++.** No JSON, YAML, TOML or DSL for project, runner or lint configuration. Lint rules are C++ callbacks; there is no clang-format or clang-tidy integration and no separate "format rule" type. A rule that calls `ctx.SetContent` is a transform, and that is the whole mechanism.
-2. **Per-project artifacts go in `<project>/build` and `<project>/bin`. Only cross-project things go in `~/.cache/crafter.build/`:** the std PCM and Crafter.Build PCMs per target and march, and external clones keyed by their spec hash.
+2. **Per-project artifacts go in `<project>/build` and `<project>/bin`. Only cross-project things go in `~/.cache/crafter.build/`:** the std PCM and Crafter.Build PCMs per target, march, toolchain and std module flags, external clones keyed by their spec hash, and the mingw sysroot views keyed by prefix.
 3. **Sources ship, PCMs build locally.** The Flatpak installs `*.cppm` under `/app/share/crafter-build/`. Nothing target-baked ships except the executable and the toolchain beside it, and the executable is built for a portable `-march`.
 4. **crafter-build is a monolithic executable with `--export-dynamic`.** `project.so` resolves `Crafter::` symbols from the running binary at dlopen time. There is no Windows host; `x86_64-w64-mingw32` is a cross-compile target for user projects only.
 5. **Convention over configuration.** Lead with the zero-config path. Env var, then CLI flag, then full `project.cpp` are the escalation, not the entry point.
@@ -189,6 +189,8 @@ Local builds are two scripts in `project.cpp`. Run them with the native binary:
 - A `#pragma crafter dependency` target is rebuilt on every load on purpose: clang crashes rather than erroring when it imports a BMI whose own imported PCM was rebuilt underneath it, and the depfile check is what catches a regenerated host cache. Do not optimise the rebuild away.
 - A host-tooling library such as Crafter.LintRules may `import Crafter.Build;` without declaring it as a dependency, because `LoadProject` fills the cache it resolves from. Listing Crafter.Build in its `cfg.dependencies` would clone and build all of crafter-build for every consumer. Its test does link crafter-build, so that test is wired behind `IsTopLevelProject()`: without the guard, crafter-build loading the library as a dependency would load its own `project.cpp` again through the test's `GitProject` and hit the cycle guard.
 - `crafter-build clean` leaves a pragma dependency's build alone; it lives with the clone in the cache.
+- Units see a std module rebuilt in place, because their depfiles list the BMIs they import (`-fmodule-file-deps`), but not one that moved to another directory: their old depfiles still name the old one. So anything that changes how the std module is compiled goes into `StdPcmFlags`, which `BuildStdPcm` compiles with and `StdPcmKey` hashes. `VariantId()` hashes `StdPcmKey`, so the build moves with it. A flag added to the command anywhere else leaves both directories where they are.
+- A CMake external whose recorded configure inputs (`<build>/.crafter-options`) differ from this build's is built again from an empty build directory, not reconfigured in place: CMake keeps what an old compiler or `--sysroot` compiled, and the check results it cached.
 - The bootstrap binary is compiled without `CRAFTER_BUILD_HAS_ASSET`, so `cfg.assets` only works in the self-hosted binary. Changing asset extensions means updating two maps in `Crafter.Build-Clang.cpp`: `assetOutput` and the transitive copy loop.
 - A warning is printed only when its unit actually recompiles. A build with nothing to do is silent, which is not evidence that a warning is gone.
 - A std module source only compiles against the headers of its own libc++ release, and a sysroot does not always ship one (Alpine). The Flatpak therefore carries the sources of the release next to the bundled one under `share/crafter-build/libc++-modules/<major>/`, one `libcxx-<major>-modules` manifest module each. When the bundled LLVM is bumped, keep the release it leaves behind as such a module: sysroots on the stable distro stay on it for months. The native loop has no such directory; stage one by hand under `share/crafter-build/` to test against a foreign sysroot.

README.md

@@ -201,7 +201,7 @@ For elaborate setups (rsync, ssh ControlMaster, env vars on the spawn, `QEMU_LD_
 
 For cross-arch builds, set `cfg.sysroot` on the `Configuration` to point at a sysroot for the target. `BuildStdPcm` and the compile/link commands both consume it: `--sysroot=<path>` is passed to clang, and the std module is precompiled against the sysroot's libc++ headers. Ordinary compiles are pointed at the same headers (`-nostdinc++ -isystem <sysroot>/usr/include/c++/v1`), so a unit that `#include`s a standard header gets the sysroot's libc++ and not the host toolchain's. The module source has to come from the same libc++ release as those headers, so it is taken from `<sysroot>/usr/share/libc++/v1/std.cppm` when the sysroot ships it, else from the toolchain when both are the same major release, else from the sources crafter-build carries for other releases (`share/crafter-build/libc++-modules/<major>/`, `crafter-build --version` lists them). Alpine is the common case for the last two: its `libc++-dev` has the headers and no module source. When nothing matches, the build stops with one line naming both releases, installing that release's `share/libc++/v1` directory into the sysroot resolves it. The cheapest aarch64 sysroot is an extracted [Arch Linux ARM rootfs](http://os.archlinuxarm.org/) plus `libc++` + `libc++abi` + `gcc` installed into it via `pacman --root <rootfs> --config <alarm-pacman.conf> --dbpath <rootfs>/var/lib/pacman -Sy …` (point pacman at ALARM's mirrors via explicit config to avoid pulling host-arch packages).
 
-For `--target=x86_64-w64-mingw32` (Windows from Linux), the pipeline cross-compiles as long as `mingw-w64` is installed, with libstdc++ 15 or newer for its std module. clang finds it by the `x86_64-w64-mingw32-gcc` on `PATH`; `CRAFTER_MINGW_DIR` names another prefix (the directory holding `x86_64-w64-mingw32/` and `lib/gcc/`), and inside the Flatpak the host's `/usr` is used when it has mingw-w64. Without one the build stops with a message saying where it looked. Pair it with `--runner=cmd:wine` to run the result. Only the MinGW ABI is supported, there is no MSVC target and no Windows host. `-fuse-ld=lld` is auto-injected on every link, and `-lstdc++exp` is auto-linked for the mingw target so user projects don't need to declare those flags.
+For `--target=x86_64-w64-mingw32` (Windows from Linux), the pipeline cross-compiles as long as `mingw-w64` is installed, with libstdc++ 15 or newer for its std module. clang finds it by the `x86_64-w64-mingw32-gcc` on `PATH`; `CRAFTER_MINGW_DIR` names another prefix (the directory holding `x86_64-w64-mingw32/` and `lib/gcc/`), and inside the Flatpak the host's `/usr` is used when it has mingw-w64. Without one the build stops with a message saying where it looked. The prefix is clang's sysroot as it is, so Windows libraries installed into its `include/` and `lib/` are found, unless it also holds a Linux system: the host's `/usr` in the Flatpak, or any prefix whose `include/` has a C library's `features.h`, as `/usr` does. clang searches a sysroot's `include/` and `lib/`, and in such a prefix those hold glibc's headers and Linux libraries, so clang is shown only its `x86_64-w64-mingw32/`, `lib/gcc/` and `lib64/gcc/`, as links in `mingw-sysroot/` in the cache (`~/.cache/crafter.build/`, in the Flatpak `~/.var/app/net.catcrafts.CrafterBuild/cache/crafter.build/`, or `$XDG_RUNTIME_DIR/crafter.build/` when the cache's filesystem takes no symbolic links). Compiler messages still name those headers by their paths in the prefix. Pair it with `--runner=cmd:wine` to run the result. Only the MinGW ABI is supported, there is no MSVC target and no Windows host. `-fuse-ld=lld` is auto-injected on every link, and `-lstdc++exp` is auto-linked for the mingw target so user projects don't need to declare those flags.
 
 For `--target=wasm32-wasip1`, the WASI sysroot is the one beside the toolchain (the Flatpak bundles one) unless `CRAFTER_BUILD_WASI_SYSROOT` names another, and tests run under `wasmtime`. C++ exceptions work as on the other targets: every unit and the std module are compiled with `-fwasm-exceptions` in the standardized form of WebAssembly exception handling (exnref), against the exception-enabled libc++ in the sysroot's `lib/<triple>/eh/`, and `-lunwind` is linked automatically. A sysroot without that libc++ (wasi-sdk before 33) stops the build with one line saying so. The result runs on engines with that proposal: Chrome 137, Firefox 131 and Safari 18.4 or newer, and the wasmtime the Flatpak bundles. A page from `EnableWasiBrowserRuntime` checks first and names what an older browser lacks in the console, instead of failing to compile the module. An exception that escapes `main` leaves the program as an uncaught WebAssembly exception (wasmtime reports `thrown Wasm exception` and exits 1, a page gets a `WebAssembly.Exception`) without calling `std::terminate`, so a `std::set_terminate` handler does not run and nothing prints the exception's type or `what()`: wasm exception handling unwinds in a single phase, and nothing finds out that no handler is left until the stack is gone. Catch it in `main` to report it.
 

implementations/Crafter.Build-Clang.cpp

@@ -247,29 +247,51 @@ void Configuration::GetInterfacesAndImplementations(std::span<fs::path> interfac
     }
 }
 
+std::string Crafter::StdPcmFlags(const Configuration& config) {
+    if (config.target == "x86_64-w64-mingw32") {
+        // -femulated-tls keeps PCM TLS access matching libstdc++'s emutls
+        // definitions; a mismatch surfaces as undefined std::__once_callable.
+        return std::format(" --target={} -march={} -mtune={} -femulated-tls -O3 -std=c++26 -Wno-reserved-identifier -Wno-reserved-module-identifier", config.target, config.march, config.mtune);
+    }
+    const bool isWasm = config.target.starts_with("wasm32");
+    const std::string sysrootFlag = config.sysroot.empty() ? std::string() : std::format(" --sysroot={}", config.sysroot);
+    // wasm32 rejects -march, and the std module has to be compiled the way
+    // the units importing it are, a variant pass's flags (e.g.
+    // -mrelaxed-simd) included, or the BMI is not feature-compatible.
+    std::string archFlags = isWasm ? WasmCompileFlags(config) : std::format(" -march={} -mtune={}", config.march, config.mtune);
+    // non-wasm cross builds: the driver ranks the host toolchain's own
+    // libc++ headers (<install>/../include/c++/v1) above --sysroot, so the
+    // PCM would mix host libc++ with the foreign libc. Force the sysroot's
+    // copy.
+    if (!isWasm && !config.sysroot.empty()) {
+        archFlags += std::format(" -nostdinc++ -isystem {}/usr/include/c++/v1", config.sysroot);
+    }
+    return std::format(" --target={} -std=c++26{}{} -O3 -Wno-reserved-identifier -Wno-reserved-module-identifier", config.target, sysrootFlag, archFlags);
+}
+
+std::string Crafter::StdPcmKey(const Configuration& config, const Toolchain& toolchain) {
+    // A PCM is only valid for the clang that wrote it and the flags it was
+    // written with, so a compiler bump starts a cold directory instead of
+    // handing the new clang a PCM it will crash on, and so does any flag.
+    // With only some of them in the name, configurations that differed in
+    // the rest took turns rebuilding one std PCM in place: -mtune=generic
+    // against project.cpp's -mtune=native on every run, two sysroots, or two
+    // releases sharing a cache, as the Flatpak's branches do, each time under
+    // the PCMs built against the previous one, which clang crashes on rather
+    // than rejecting.
+    std::string compiled = StdPcmFlags(config);
+    // The base command's part: the toolchain's key covers the compiler and
+    // libc++, and for the mingw target what clang finds mingw-w64 through is
+    // the prefix, given whole or through a view (MingwSysroot), named here by
+    // the prefix rather than by the view's own path.
+    if (config.target == "x86_64-w64-mingw32" && !toolchain.mingwPrefix.empty()) {
+        compiled += std::format("|mingw:{}|{}", toolchain.mingwPrefix.string(), toolchain.mingwPrefixIsSystem ? "view" : "whole");
+    }
+    return std::format("{}-{}-{}-{:08x}", config.target, config.march, toolchain.Key(), static_cast<std::uint32_t>(std::hash<std::string>{}(compiled)));
+}
+
 fs::path Crafter::StdPcmDir(const Configuration& config) {
-    // The std PCM is cached per target+march+toolchain: a PCM is only valid
+    return GetCacheDir() / StdPcmKey(config, GetToolchain());
-    // for the exact clang that wrote it, so a compiler bump starts a cold
-    // directory instead of handing the new clang a PCM it will crash on. A
-    // wasm variant pass compiles it with extra codegen flags (e.g.
-    // -mrelaxed-simd) — its BMI must not be shared with the baseline's, or
-    // the consuming TUs see a target-feature mismatch. Suffix the cache dir
-    // with the variant flags when present.
-    std::string stdPcmKey = std::format("{}-{}-{}", config.target, config.march, GetToolchain().Key());
-    // The flags every wasm unit gets are compiled into the std PCM as well,
-    // and the importing units have to match them. Two releases that share a
-    // cache and a compiler, as the Flatpak's branches do, would otherwise
-    // take turns rebuilding this one std PCM, each time underneath the PCMs
-    // built against the previous one, and clang crashes on those rather than
-    // rejecting them.
-    if (config.target.starts_with("wasm32")) {
-        stdPcmKey += std::format("-{:08x}", static_cast<std::uint32_t>(std::hash<std::string_view>{}(WasmBaselineFlags)));
-    }
-    for (const std::string& f : config.wasmVariantFlags) {
-        stdPcmKey += "+";
-        for (char c : f) stdPcmKey += (c == '/' || c == '\\') ? '_' : c;
-    }
-    return GetCacheDir()/stdPcmKey;
 }
 
 std::string Crafter::WasmCompileFlags(const Configuration& config) {
@@ -928,9 +950,9 @@ BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, s
         : std::format(" -march={} -mtune={}", config.march, config.mtune);
     if (!config.sysroot.empty()) {
         cArchFlags += std::format(" --sysroot={}", config.sysroot);
-    } else if (config.target == "x86_64-w64-mingw32" && !GetToolchain().mingwSysroot.empty()) {
+    } else if (config.target == "x86_64-w64-mingw32") {
         // The C++ side gets it through GetBaseCommand.
-        cArchFlags += std::format(" --sysroot={}", GetToolchain().mingwSysroot.string());
+        if (const fs::path& sysroot = MingwSysroot(); !sysroot.empty()) cArchFlags += std::format(" --sysroot={}", sysroot.string());
     }
     if (isWasm) {
         // The C++ path's wasm flags, all of them, a variant pass's included.
@@ -1868,9 +1890,11 @@ Environment:
   CRAFTER_BUILD_LIBCLANG               Exact libclang to load for lint.
   CRAFTER_BUILD_WASI_SYSROOT           WASI sysroot for wasm32 targets.
   CRAFTER_MINGW_DIR                    mingw-w64 prefix (holding x86_64-w64-mingw32/
-                                       and lib/gcc/), passed as --sysroot. Default:
+                                       and lib/gcc/). Default: the mingw gcc on
-                                       the mingw gcc on PATH; in the Flatpak, the
+                                       PATH; in the Flatpak, the host's /usr when
-                                       host's /usr when it has mingw-w64.
+                                       it has mingw-w64. Of a prefix that holds a
+                                       Linux system too, such as /usr, clang sees
+                                       only those directories.
   CRAFTER_BUILD_HOME                   Directory holding the module sources.
 
 Exit status:

implementations/Crafter.Build-External.cpp

@@ -149,7 +149,7 @@ std::string BuildInjectedCMakeFlags(const fs::path& cmakeBuildDir, std::string_v
         // The libstdc++ and the mingw-w64 install the project's own compiles
         // use (GetBaseCommand).
         out += " -DCMAKE_CXX_FLAGS=-stdlib=libstdc++";
-        if (!tc.mingwSysroot.empty()) out += std::format(" -DCMAKE_SYSROOT={}", ShellQuote(tc.mingwSysroot.string()));
+        if (const fs::path& sysroot = MingwSysroot(); !sysroot.empty()) out += std::format(" -DCMAKE_SYSROOT={}", ShellQuote(sysroot.string()));
     }
     fs::path absBuildDir = fs::absolute(cmakeBuildDir);
     out += std::format(" -DCMAKE_ARCHIVE_OUTPUT_DIRECTORY={}", ShellQuote(absBuildDir.string()));
@@ -179,6 +179,19 @@ std::string ConfigureCMake(const fs::path& cloneDir, const fs::path& cmakeBuildD
 
     if (!needsConfigure) return "";
 
+    // A build directory configured with other inputs starts over instead of
+    // being reconfigured. CMake reconfigures in place without recompiling
+    // what an old compiler or --sysroot compiled (the Makefiles generator
+    // writes --sysroot into build.make, which make does not compare) and
+    // keeps the check results the old configure cached. A mingw external
+    // built against the host's /usr as the sysroot (issue #43) kept its
+    // objects reading __libc_single_threaded after the fix that way. The
+    // directory keeps its name, which projects point generated headers at.
+    if (fs::exists(cmakeBuildDir / "CMakeCache.txt")) {
+        std::error_code ec;
+        fs::remove_all(cmakeBuildDir, ec);
+        if (ec) return std::format("cannot clear {} for a fresh configure: {}", cmakeBuildDir.string(), ec.message());
+    }
     if (!fs::exists(cmakeBuildDir)) fs::create_directories(cmakeBuildDir);
 
     std::string cmd = std::format("cmake -S {} -B {}{}", ShellQuote(fs::absolute(cloneDir).string()), ShellQuote(fs::absolute(cmakeBuildDir).string()), injected);

implementations/Crafter.Build-Implementation.cpp

@@ -51,8 +51,13 @@ namespace Crafter {
             return;
         }
 
-        // -MD leaves <name>_impl.o.d beside the object for Check to read.
+        // -MD leaves <name>_impl.o.d beside the object for Check to read, and
-        std::string result = RunBuildCommand(std::format("{0} {1}.cpp -c -MD -MF {2}_impl.o.d -o {2}_impl.o", clang, path.string(), (buildDir/path.filename()).string()));
+        // -fmodule-file-deps puts the BMIs the unit imported in it with the
+        // headers. The driver adds that for --precompile only, so a std
+        // module rebuilt in place, for a mingw-w64 or libc++ update under
+        // the same compiler, left the objects importing it as they were, and
+        // the next build only relinked them.
+        std::string result = RunBuildCommand(std::format("{0} {1}.cpp -c -MD -MF {2}_impl.o.d -fmodule-file-deps -o {2}_impl.o", clang, path.string(), (buildDir/path.filename()).string()));
 
         bool expected = false;
         if(!result.empty() && buildCancelled.compare_exchange_strong(expected, true)) {

implementations/Crafter.Build-Lint.cpp

@@ -1149,6 +1149,10 @@ format` applies it to disk, and `crafter-build lint` reports where it would.
     // command per configuration — GetCompileCommand walks the dependency tree.
     const bool anyRuleNeedsAst = std::ranges::any_of(rules, [](const LintRule* r) { return r->needsAst; });
     std::unordered_map<const Configuration*, std::string> commands;
+    // Why a configuration of the build graph has no command: its files
+    // cannot be parsed, which fails the run. Cleared to nothing, the command
+    // read as "not a translation unit" and every semantic rule passed them.
+    std::unordered_map<const Configuration*, std::string> commandErrors;
     if (anyRuleNeedsAst && !opts.noAst) {
         for (Configuration* c : localConfigs) {
             auto ensure = [&](const Configuration& cfg) {
@@ -1166,10 +1170,14 @@ format` applies it to disk, and `crafter-build lint` reports where it would.
                         Progress::Task task(std::format("Building std PCM ({}-{})", cfg.target, cfg.march));
                         fs::create_directories(assembled.stdPcmDir);
                         std::string error = BuildStdPcm(cfg, assembled.stdPcmDir/"std.pcm");
-                        if (!error.empty()) command.clear();
+                        if (!error.empty()) {
+                            command.clear();
+                            commandErrors.emplace(&cfg, std::format("building the std module for {}: {}", cfg.target, error));
                         }
-                } catch (const std::exception&) {
+                    }
-                    command.clear();  // surfaced per file as an AST reason
+                } catch (const std::exception& e) {
+                    command.clear();
+                    commandErrors.emplace(&cfg, std::format("assembling the compile command for {}: {}", cfg.target, e.what()));
                 }
                 commands.emplace(&cfg, std::move(command));
             };
@@ -1200,6 +1208,11 @@ format` applies it to disk, and `crafter-build lint` reports where it would.
             // decides what is safe to touch. Skip it and make the run fail.
             if (rule->needsAst) {
                 if (opts.noAst) continue;
+                if (auto failed = commandErrors.find(owner); failed != commandErrors.end()) {
+                    astFailures.emplace(file, failed->second);
+                    skippedAstRules.insert(rule->name);
+                    continue;
+                }
                 // No compile command means this file is not a translation unit
                 // of the build graph — project.cpp, which LoadProject compiles
                 // with its own flags, or a header. A semantic rule does not

implementations/Crafter.Build-Platform.cpp

@@ -281,6 +281,9 @@ bool Crafter::MatchAny(std::span<const std::string> globs, std::string_view name
 std::string Crafter::RunBuildCommand(std::string_view cmd) {
     Progress::EchoCommand(cmd);
     CommandResult r = RunCommandChecked(cmd);
+    // A mingw build against a system prefix sees mingw-w64 through a view in
+    // the cache, and clang names every header by the path it found it at.
+    r.output = MingwViewToPrefix(std::move(r.output));
 
     if (r.exitCode == 0) {
         // Warnings from a step that succeeded. Show them — until now they were
@@ -737,14 +740,17 @@ std::string Crafter::BuildStdPcm(const Configuration& config, fs::path stdPcm) {
         // library, naming the std module's source relative to itself. It is
         // looked for in the library directories the compiler searches for
         // this target, as it would look for libstdc++ itself, but only in
-        // those that belong to the mingw toolchain: the list also has the
+        // those that belong to the mingw toolchain. The list also holds
-        // generic <prefix>/lib, where the host's own (Linux) libstdc++ keeps
+        // clang's resource directory, the sysroot's generic lib/, where a
-        // its manifest. A toolchain that isn't there is a build error, not
+        // prefix given whole can keep a libstdc++ built for another target,
-        // an exception: Build() runs this with its worker threads started.
+        // and, when clang finds no gcc directory, an empty entry that would
+        // name a manifest in the working directory. A toolchain that isn't
+        // there is a build error, not an exception: Build() runs this with
+        // its worker threads started.
         const Toolchain& tc = GetToolchain();
-        const std::string where = tc.mingwSysroot.empty()
+        const std::string where = tc.mingwPrefix.empty()
             ? std::string("where clang finds it by the x86_64-w64-mingw32 gcc on PATH")
-            : std::format("under {}", tc.mingwSysroot.string());
+            : std::format("under {}", tc.mingwPrefix.string());
         const std::string missing = std::format("no mingw-w64 libstdc++ with the std module (libstdc++ 15 or newer) {}. Install mingw-w64, or point CRAFTER_MINGW_DIR at its prefix, the directory holding x86_64-w64-mingw32/ and lib/gcc/", where);
         // Asked once a run for each compiler and target: every mingw
         // configuration comes through here, up to date or not, and the answer
@@ -775,17 +781,19 @@ std::string Crafter::BuildStdPcm(const Configuration& config, fs::path stdPcm) {
             }
         }
         if (manifestPath.empty()) return missing;
+        // Found through the view MingwSysroot() may have made of the prefix,
+        // and named from here on by its place in the prefix, the directory
+        // the user knows and can go and look at.
+        manifestPath = fs::path(MingwViewToPrefix(manifestPath.string()));
         const std::optional<std::string> source = StdModuleSourcePath(FileBytes(manifestPath));
         if (!source) return std::format("{} names no std module", manifestPath.string());
         const fs::path stdCc = (manifestPath.parent_path() / *source).lexically_normal();
         if (!fs::exists(stdCc)) return std::format("{}, which {} names as the std module, is missing", stdCc.string(), manifestPath.string());
 
-        // -femulated-tls keeps PCM TLS access matching libstdc++'s emutls
-        // definitions; mismatch surfaces as undefined std::__once_callable.
         // The std.cc → std.cppm copy uses a plain filename: held under the
         // cache lock, no other builder writes this path concurrently.
         fs::path stdCppm = stdPcm.parent_path() / "std.cppm";
-        std::string command = std::format("{} --target={} -march={} -mtune={} -femulated-tls -O3 -std=c++26 -Wno-reserved-identifier -Wno-reserved-module-identifier --precompile {} -o {}", GetBaseCommand(config), config.target, config.march, config.mtune, stdCppm.string(), stdPcm.string());
+        std::string command = std::format("{}{} --precompile {} -o {}", GetBaseCommand(config), StdPcmFlags(config), stdCppm.string(), stdPcm.string());
         std::string stamp = HashStamp(command + '\0' + FileBytes(stdCc));
         if (fs::exists(stdPcm) && ReadStamp(StampPath(stdPcm)) == stamp) return "";
         std::error_code copyEc;
@@ -815,27 +823,11 @@ std::string Crafter::BuildStdPcm(const Configuration& config, fs::path stdPcm) {
             if (!source.error.empty()) return source.error;
             stdCppm = source.stdCppm.string();
         }
-        std::string sysrootFlag = config.sysroot.empty()
-            ? std::string()
-            : std::format(" --sysroot={}", config.sysroot);
-        // wasm32 rejects -march, and the std module has to be compiled the way
-        // the units importing it are, a variant pass's flags (e.g.
-        // -mrelaxed-simd) included, or the BMI is not feature-compatible.
-        std::string archFlags = isWasm
-            ? WasmCompileFlags(config)
-            : std::format(" -march={} -mtune={}", config.march, config.mtune);
-        // non-wasm cross builds: the driver ranks the host toolchain's own
-        // libc++ headers (<install>/../include/c++/v1) above --sysroot, so
-        // the PCM would mix host libc++ with the foreign libc — force the
-        // sysroot's copy.
-        if (!isWasm && !config.sysroot.empty()) {
-            archFlags += std::format(" -nostdinc++ -isystem {}/usr/include/c++/v1", config.sysroot);
-        }
         // Keyed on the command and the module source's bytes, never on
         // mtimes: an installed std.cppm can be dated at the epoch (OSTree),
         // and a libc++ update under the same clang changes the bytes but
         // not the path.
-        std::string command = std::format("{} --target={} -std=c++26{}{} -O3 -Wno-reserved-identifier -Wno-reserved-module-identifier --precompile {} -o {}", GetBaseCommand(config), config.target, sysrootFlag, archFlags, stdCppm, stdPcm.string());
+        std::string command = std::format("{}{} --precompile {} -o {}", GetBaseCommand(config), StdPcmFlags(config), stdCppm, stdPcm.string());
         std::string stamp = HashStamp(command + '\0' + FileBytes(stdCppm));
         if (fs::exists(stdPcm) && ReadStamp(StampPath(stdPcm)) == stamp) return "";
         std::string result = RunBuildCommand(command);
@@ -862,9 +854,9 @@ std::string Crafter::GetBaseCommand(const Configuration& config) {
     if (config.target == "x86_64-w64-mingw32") {
         // libstdc++ from the mingw toolchain, said outright: a clang built to
         // default to libc++ (the Flatpak's) would otherwise reach for that.
-        // Found by clang's own mingw discovery unless there is a sysroot.
+        // Found by clang's own mingw discovery unless there is a prefix.
         command += " -stdlib=libstdc++";
-        if (!tc.mingwSysroot.empty()) command += std::format(" --sysroot={}", tc.mingwSysroot.string());
+        if (const fs::path& sysroot = MingwSysroot(); !sysroot.empty()) command += std::format(" --sysroot={}", sysroot.string());
         return command;
     }
     command += " -stdlib=libc++";

implementations/Crafter.Build-Toolchain.cpp

@@ -43,6 +43,37 @@ namespace {
         while (!s.empty() && (s.back() == '\n' || s.back() == '\r' || s.back() == ' ')) s.pop_back();
         return s;
     }
+
+    // `link` made a symbolic link to `target`, or found one already. Another
+    // crafter-build may make the same link at the same moment, which is as
+    // good as making it; a link to anything else is an error.
+    void EnsureSymlink(const fs::path& target, const fs::path& link) {
+        std::error_code ec;
+        if (fs::read_symlink(link, ec) == target) return;
+        fs::create_directories(link.parent_path());
+        fs::create_directory_symlink(target, link, ec);
+        std::error_code ignored;
+        if (ec && fs::read_symlink(link, ignored) != target) {
+            throw fs::filesystem_error("cannot make a link", target, link, ec);
+        }
+    }
+
+    // `dir` made absolute and spelled one way, once, when the toolchain is
+    // resolved. The mingw prefix ends up as the target of links and in the
+    // names of build and cache directories: a relative path read later from
+    // another working directory would name something else, and /opt/mingw/
+    // next to /opt/mingw would rebuild everything for the same directory.
+    fs::path PrefixPath(const fs::path& dir) {
+        fs::path normal = fs::absolute(dir).lexically_normal();
+        if (!normal.has_filename() && normal.has_relative_path()) normal = normal.parent_path();
+        return normal;
+    }
+
+    // The view MingwSysroot() made and the prefix it shows, for
+    // MingwViewToPrefix. Written once, before ViewMade is set.
+    fs::path MadeView;
+    fs::path ViewedPrefix;
+    std::atomic<bool> ViewMade = false;
 }
 
 std::string Toolchain::LibcxxCompileFlags() const {
@@ -136,11 +167,19 @@ Toolchain Crafter::ResolveToolchain(const ToolchainInputs& in) {
         tc.wasiSysroot = "/usr/share/wasi-sysroot";
     }
     if (std::string_view dir = Env(in, "CRAFTER_MINGW_DIR"); !dir.empty()) {
-        tc.mingwSysroot = fs::path(dir);
+        tc.mingwPrefix = PrefixPath(fs::path(dir));
+        // A prefix that holds a Linux system as well as mingw-w64, /usr
+        // itself, has that system's C library in include/: features.h, which
+        // every glibc and musl header includes and mingw-w64 has no use for.
+        // One that holds mingw-w64 alone stays the sysroot, so Windows
+        // libraries installed into its include/ and lib/ stay visible.
+        tc.mingwPrefixIsSystem = fs::exists(tc.mingwPrefix / "include" / "features.h");
     } else if (!in.hostRoot.empty() && fs::exists(in.hostRoot / "usr" / "x86_64-w64-mingw32")) {
         // The sandbox has no mingw-w64 of its own, and no mingw gcc on PATH
-        // for clang to find one by.
+        // for clang to find one by. The host's /usr is the host's system,
-        tc.mingwSysroot = in.hostRoot / "usr";
+        // whether or not its C library's headers are installed.
+        tc.mingwPrefix = PrefixPath(in.hostRoot / "usr");
+        tc.mingwPrefixIsSystem = true;
     }
     return tc;
 }
@@ -186,6 +225,69 @@ const Toolchain& Crafter::RequireToolchain() {
     return tc;
 }
 
+fs::path Crafter::MakeMingwSysroot(const fs::path& prefix, std::span<const fs::path> parents) {
+    std::string reasons;
+    for (const fs::path& parent : parents) {
+        // One view per prefix: two shells with different CRAFTER_MINGW_DIRs
+        // share one cache and would otherwise rewrite each other's.
+        const fs::path sysroot = parent / "mingw-sysroot" / std::format("{:016x}", std::hash<std::string>{}(prefix.string()));
+        try {
+            // All clang's mingw driver takes from a sysroot other than its
+            // include/ and the rest of its lib/: the mingw-w64 tree (headers,
+            // libraries, and libstdc++ where a distro's cross gcc installs
+            // it) and gcc's per-target directory (crtbegin.o, libgcc, and the
+            // version that names libstdc++'s header directory), which
+            // openSUSE keeps under lib64. A link to nothing is skipped like a
+            // missing directory.
+            for (std::string_view entry : {"x86_64-w64-mingw32", "lib/gcc", "lib64/gcc"}) {
+                EnsureSymlink(prefix / entry, sysroot / entry);
+            }
+            return sysroot;
+        } catch (const std::exception& e) {
+            reasons += std::format("; {}", e.what());
+        }
+    }
+    throw std::runtime_error(std::format("cannot make a sysroot that shows clang only the mingw-w64 parts of {}{}", prefix.string(), reasons));
+}
+
+const fs::path& Crafter::MingwSysroot() {
+    // Made on first use rather than by GetToolchain(), so that only what
+    // compiles for the mingw target, a build or an AST lint of one, writes
+    // anything for it, and a failure surfaces as that command's error
+    // instead of on every command.
+    static const fs::path Sysroot = []() -> fs::path {
+        const Toolchain& tc = GetToolchain();
+        if (!tc.mingwPrefixIsSystem) return tc.mingwPrefix;
+        // In the cache, or where the XDG spec promises symbolic links when
+        // the cache's filesystem takes none (vfat, exFAT, CIFS without unix
+        // extensions), which costs nothing else crafter-build caches.
+        // Absolute, because CMake runs the compiler from directories of its
+        // own.
+        std::vector<fs::path> parents = { fs::absolute(GetCacheDir()) };
+        if (const char* runtime = std::getenv("XDG_RUNTIME_DIR"); runtime && *runtime) {
+            parents.push_back(fs::absolute(fs::path(runtime) / "crafter.build"));
+        }
+        fs::path view = MakeMingwSysroot(tc.mingwPrefix, parents);
+        MadeView = view;
+        ViewedPrefix = tc.mingwPrefix;
+        ViewMade.store(true, std::memory_order_release);
+        return view;
+    }();
+    return Sysroot;
+}
+
+std::string Crafter::MingwViewToPrefix(std::string text) {
+    if (!ViewMade.load(std::memory_order_acquire)) return text;
+    // With the separator, so a --sysroot= naming the view itself, in a
+    // command line the output quotes, stays what was run.
+    const std::string view = (MadeView / "").string();
+    const std::string prefix = (ViewedPrefix / "").string();
+    for (std::size_t at = text.find(view); at != std::string::npos; at = text.find(view, at + prefix.size())) {
+        text.replace(at, view.size(), prefix);
+    }
+    return text;
+}
+
 std::int32_t Crafter::LibcxxVersion(const fs::path& includeDir) {
     // "#  define _LIBCPP_VERSION 230102", the one place libc++ states its
     // release in a form the preprocessor does not have to be run for.

interfaces/Crafter.Build-Clang.cppm

@@ -392,6 +392,15 @@ export namespace Crafter {
     };
 
     struct Configuration;
+    // The name StdPcmDir gives the directory of this configuration's std
+    // module under `toolchain`: target, march and the toolchain's key, which
+    // say what it is, then a hash of everything else it is compiled with,
+    // each flag BuildStdPcm passes (StdPcmFlags) and for the mingw target the
+    // prefix behind its --sysroot. Two configurations that would compile the
+    // std module differently never share a directory, so neither rebuilds it
+    // under the PCMs the other compiled against it. Declared ahead of
+    // Configuration, whose VariantId keys on it.
+    std::string StdPcmKey(const Configuration& config, const Toolchain& toolchain);
 
     // What a Script's callback is handed when `crafter-build run <name>`
     // starts it.
@@ -580,7 +589,8 @@ export namespace Crafter {
         void AddAstLintRule(std::string name, std::function<void(LintContext&)> check);
         // Suffix that uniquely identifies this Configuration's compile state.
         // target+march+mtune are spelled out for readability; the rest
-        // (type, debug, sysroot, defines, compileFlags, the toolchain)
+        // (type, debug, sysroot, defines, compileFlags, the std module, the
+        // toolchain)
         // collapse into a short hash so two Configurations sharing a path but
         // with different compile state can't clobber each other's outputs.
         std::string VariantId() const { return VariantId(GetToolchain()); }
@@ -628,6 +638,15 @@ export namespace Crafter {
                 compileKey += "|B:";
                 compileKey += WasmBaselineFlags;
             }
+            // The std module the units import. Its PCMs and objects record
+            // it by path, and units do not notice it moving: when the mingw
+            // prefix stopped being clang's sysroot as is (issue #43), the
+            // std module was rebuilt and the objects compiled against the
+            // host's glibc headers were only relinked, failing as before. A
+            // std module that moves directories, because anything it is
+            // compiled with changed, moves the build with it.
+            compileKey += "|S:";
+            compileKey += StdPcmKey(*this, toolchain);
             // Sorted by ApplyStandardArgs, so flag order on the command line
             // doesn't split the cache.
             for (const std::string& a : projectArgs) {
@@ -662,10 +681,9 @@ export namespace Crafter {
         std::vector<std::string> requires_;
     };
 
-    // Directory holding the std module PCM for this Configuration. Keyed on
+    // Directory holding the std module PCM for this Configuration,
-    // target+march because a BMI is only loadable by a TU with matching target
+    // GetCacheDir() / StdPcmKey(config, GetToolchain()): a BMI is only
-    // features, and suffixed with the wasm flags (WasmBaselineFlags, then the
+    // loadable by a unit compiled the way it was.
-    // variant's) for the same reason.
     fs::path StdPcmDir(const Configuration& config);
 
     // Where Build leaves this Configuration's linked artifact: the executable
@@ -794,4 +812,9 @@ namespace Crafter {
     // compile command and of the std module's, which have to agree flag for
     // flag.
     std::string WasmCompileFlags(const Configuration& config);
+    // The flags BuildStdPcm compiles the std module with, after the base
+    // command and before the source, each with a leading space. StdPcmKey
+    // hashes the same string, so the command and the directory it writes to
+    // cannot disagree about what decides the BMI.
+    std::string StdPcmFlags(const Configuration& config);
 }

interfaces/Crafter.Build-Toolchain.cppm

@@ -39,11 +39,20 @@ export namespace Crafter {
         fs::path wasiSysroot;
         // The mingw-w64 install prefix, the directory that holds
         // x86_64-w64-mingw32/ and lib/gcc/x86_64-w64-mingw32/ (/usr for a
-        // distro package), passed to clang as --sysroot for the mingw target.
+        // distro package), absolute and lexically normal. Empty means clang's
-        // Empty means clang's own discovery, which follows the mingw gcc on
+        // own discovery, which follows the mingw gcc on PATH. Set by
-        // PATH. Set by CRAFTER_MINGW_DIR, else the host's /usr when running
+        // CRAFTER_MINGW_DIR, else the host's /usr when running sandboxed and
-        // sandboxed and it has mingw-w64 (the sandbox's own /usr never does).
+        // it has mingw-w64 (the sandbox's own /usr never does). What the
-        fs::path mingwSysroot;
+        // mingw target passes as --sysroot is MingwSysroot(), not this:
+        // see mingwPrefixIsSystem.
+        fs::path mingwPrefix;
+        // The prefix holds a Linux system as well as mingw-w64: the host's
+        // /usr in the sandbox, or a prefix whose include/ has a C library's
+        // features.h, as /usr has. clang searches a mingw sysroot's include/
+        // and lib/, so such a prefix is shown to it only through a view of
+        // its mingw-w64 parts; any other prefix is the sysroot itself, and
+        // Windows libraries installed into its include/ and lib/ are found.
+        bool mingwPrefixIsSystem = false;
         // `clang++ -print-target-triple` and the first line of
         // `clang++ --version`; both empty when clang++ could not be run.
         std::string hostTriple;
@@ -86,6 +95,32 @@ export namespace Crafter {
     const Toolchain& GetToolchain();
     // GetToolchain(), or a std::runtime_error carrying its error.
     const Toolchain& RequireToolchain();
+    // Makes, in the first of `parents` that can hold one, a directory that
+    // shows clang only the mingw-w64 parts of `prefix`: links named
+    // x86_64-w64-mingw32, lib/gcc and lib64/gcc to the same paths under
+    // `prefix`, at <parent>/mingw-sysroot/<hash of prefix>, and returns it.
+    // A view already there is reused; one another process makes at the same
+    // moment is as good as one made here. Throws when no parent can hold
+    // one, with the reason for each.
+    fs::path MakeMingwSysroot(const fs::path& prefix, std::span<const fs::path> parents);
+    // The --sysroot of every compile, link and CMake configure for the mingw
+    // target, settled on the first call of a process: empty without a
+    // mingwPrefix, the prefix itself for one that holds mingw-w64 alone, and
+    // for a system prefix the view MakeMingwSysroot makes in crafter-build's
+    // cache, or in $XDG_RUNTIME_DIR/crafter.build when the cache's
+    // filesystem takes no symbolic links. A system prefix cannot be the
+    // sysroot: clang takes a mingw sysroot for a tree that holds mingw-w64
+    // alone and searches its include/ and lib/ as well, which for /usr are
+    // the Linux system's own. A Windows build then saw glibc's headers, and
+    // libstdc++, finding <sys/single_threaded.h>, had shared_ptr read
+    // __libc_single_threaded, which nothing on Windows defines (issue #43).
+    // Throws when the view cannot be made.
+    const fs::path& MingwSysroot();
+    // `text` with every path into the view MingwSysroot() made written as
+    // the same path into the prefix it shows, so that compiler messages name
+    // a header where the user installed it rather than in a hashed cache
+    // directory; `text` unchanged when no view was made.
+    std::string MingwViewToPrefix(std::string text);
     // _LIBCPP_VERSION as the <__config> in `includeDir` (an include/c++/v1)
     // defines it, XXYYZZ; 0 when the file is missing or has no such line.
     std::int32_t LibcxxVersion(const fs::path& includeDir);

project.cpp

@@ -247,15 +247,18 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
         cfg.AddTest("IncrementalInterfaceChange").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("TransitiveInterfaceChange").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("IncrementalHeaderChange").Dependencies({ CrafterBuildLib.get() });
+        cfg.AddTest("StdPcmRebuild").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("CompilerWarning").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("CleanProject").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("ShaderCompile").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("ShaderVariantBuild").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("ShaderIncludeChange").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("MingwMissing").Dependencies({ CrafterBuildLib.get() });
+        cfg.AddTest("MingwSysroot").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("GitSubmodules").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("LinkFlagsChange").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("ExternalArtifactTime").Dependencies({ CrafterBuildLib.get() });
+        cfg.AddTest("ExternalReconfigure").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("ConcurrentExternalFetch").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("SharedExternalFetch").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("StandardArgs").Dependencies({ CrafterBuildLib.get() });

tests/ExternalReconfigure/main.cpp

@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// A CMake dependency configured with other inputs than this build's (another
+// compiler or --sysroot, the flags crafter-build injects, which it records in
+// <build>/.crafter-options) has to be built again from nothing. It was
+// reconfigured in place, and CMake does not recompile what a changed
+// --sysroot compiled (the Makefiles generator writes it into build.make,
+// which make does not compare) and keeps the check results of the old
+// configure. A mingw dependency built by a release that gave clang the
+// host's /usr as the sysroot (issue #43) kept its objects reading
+// __libc_single_threaded after the update.
+//
+// The dependency is a local repository with a one-file C library. A file
+// planted in its build directory stands for what the old configure left,
+// and the recorded inputs are rewritten the way another crafter-build would
+// have left them. Everything is local; XDG_CACHE_HOME keeps the clone out of
+// the real cache.
+
+extern "C" int setenv(const char* name, const char* value, int overwrite);
+
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+
+    void Git(const fs::path& dir, std::string_view args) {
+        const std::string command = std::format("git -C {} {}", ShellQuote(dir.string()), args);
+        auto result = RunCommandWithTimeout(command, std::chrono::seconds(60));
+        if (result.exitCode != 0) throw std::runtime_error(std::format("{} failed: {}", command, result.output));
+    }
+
+    void Write(const fs::path& file, std::string_view text) {
+        std::ofstream(file, std::ios::binary | std::ios::trunc) << text;
+    }
+}
+
+int main() {
+    const fs::path scratch = fs::temp_directory_path() / "crafter-build-external-reconfigure";
+    fs::remove_all(scratch);
+    fs::create_directories(scratch);
+    setenv("XDG_CACHE_HOME", (scratch / "cache").c_str(), 1);
+    setenv("GIT_CONFIG_NOSYSTEM", "1", 1);
+    setenv("GIT_CONFIG_GLOBAL", "/dev/null", 1);
+    for (const char* name : { "GIT_AUTHOR_NAME", "GIT_COMMITTER_NAME" }) setenv(name, "crafter-build test", 1);
+    for (const char* name : { "GIT_AUTHOR_EMAIL", "GIT_COMMITTER_EMAIL" }) setenv(name, "test@example.invalid", 1);
+
+    const fs::path repo = scratch / "reconf";
+    fs::create_directories(repo);
+    Write(repo / "lib.c", "int Reconfigured(void) { return 1; }\n");
+    Write(repo / "CMakeLists.txt",
+          "cmake_minimum_required(VERSION 3.16)\n"
+          "project(reconf C)\n"
+          "add_library(reconf STATIC lib.c)\n");
+    Git(repo, "init -q -b main");
+    Git(repo, "add .");
+    Git(repo, "commit -q -m reconf");
+
+    ExternalDependency dep;
+    dep.name = "reconf";
+    dep.source.url = std::format("file://{}", repo.string());
+    dep.builder = ExternalBuilder::CMake;
+    dep.libs = { "reconf" };
+    std::string error = BuildExternal(dep, HostTarget());
+    Check(error.empty(), std::format("the dependency builds: {}", error));
+
+    const fs::path build = ExternalBuildDir(dep, HostTarget());
+    const fs::path leftover = build / "left-by-the-old-configure";
+    Write(leftover, "");
+    error = BuildExternal(dep, HostTarget());
+    Check(error.empty(), std::format("and builds again: {}", error));
+    Check(fs::exists(leftover), "with the same inputs its build directory is kept");
+
+    Write(build / ".crafter-options", " -DCMAKE_SYSROOT=/usr\n");
+    error = BuildExternal(dep, HostTarget());
+    Check(error.empty(), std::format("configured with other inputs, it builds: {}", error));
+    Check(!fs::exists(leftover), "from an empty build directory");
+    Check(fs::exists(build / "libreconf.a"), "and the library is built again");
+
+    std::error_code ec;
+    fs::remove_all(scratch, ec);
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}

tests/Lint/main.cpp

@@ -6,6 +6,9 @@ import Crafter.Build;
 namespace fs = std::filesystem;
 using namespace Crafter;
 
+extern "C" int setenv(const char* name, const char* value, int overwrite);
+extern "C" int unsetenv(const char* name);
+
 namespace {
     std::int32_t Failures = 0;
 
@@ -705,6 +708,31 @@ int main() {
         // two assertions above cover it.
     }
 
+    // A configuration whose std module cannot be built is in the build graph
+    // all the same: its files cannot be parsed, which has to fail the run.
+    // Its command was cleared, which read as "not a translation unit", and
+    // every semantic rule passed the files. A sysroot without libc++ is one
+    // way there. The std module's directory is made before its build fails,
+    // so the cache is redirected around this run.
+    {
+        Scratch s("ast-no-std-module");
+        s.Write("f", "int Value = 1;\n");
+        Configuration cfg = s.Config({"f"});
+        cfg.sysroot = (s.dir / "empty-sysroot").string();
+        fs::create_directories(cfg.sysroot);
+        bool ran = false;
+        cfg.AddAstLintRule("needs-ast", [&ran](LintContext&) { ran = true; });
+        const char* realCache = std::getenv("XDG_CACHE_HOME");
+        const std::string saved = realCache ? realCache : "";
+        setenv("XDG_CACHE_HOME", (s.dir / "cache").c_str(), 1);
+        LintSummary summary = RunLint(cfg, Mode(LintMode::Report));
+        if (saved.empty()) unsetenv("XDG_CACHE_HOME");
+        else setenv("XDG_CACHE_HOME", saved.c_str(), 1);
+        Check(!ran, "ast: rule is skipped when the std module cannot be built");
+        Check(summary.errors > 0, "ast: a std module that cannot be built fails the run");
+        Check(!summary.Clean(), "ast: a std module that cannot be built is not Clean");
+    }
+
     // --no-ast skips those rules deliberately and exits normally.
     {
         Scratch s("ast-optout");

tests/MingwSysroot/fixture/cprobe.c

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// probe.cpp's checks for the C sources, which are compiled by a command of
+// their own.
+#if __has_include(<sys/single_threaded.h>)
+#error the include/ of the prefix is on the C include path: a Windows build sees the headers of the Linux system
+#endif
+#if !__has_include(<_mingw.h>)
+#error the mingw-w64 headers are not on the C include path
+#endif
+
+void CProbe(void) {}

tests/MingwSysroot/fixture/failing.cpp

@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// Stops in a header of the stand-in mingw-w64, so that the error clang
+// reports is located in one.
+#include <crafter_probe_error.h>

tests/MingwSysroot/fixture/probe.cpp

@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// Built for x86_64-w64-mingw32 against the prefix tests/MingwSysroot stages:
+// a Linux system's /usr with a mingw-w64 cross toolchain in it.
+#if __has_include(<sys/single_threaded.h>)
+#error the include/ of the prefix is on the include path: a Windows build sees the headers of the Linux system
+#endif
+#if !__has_include(<_mingw.h>)
+#error the mingw-w64 headers are not on the include path
+#endif
+#if !__has_include(<memory>)
+#error the libstdc++ headers are not on the include path: clang found no gcc version under lib/gcc of the prefix
+#endif
+
+void Probe() {}

tests/MingwSysroot/main.cpp

@@ -0,0 +1,151 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// A mingw-w64 prefix that is a whole Linux system's /usr must not hand that
+// system's headers to a Windows build. Inside the Flatpak the prefix is the
+// host's /usr, and clang got it as the --sysroot. clang's mingw driver takes
+// a sysroot for a tree that holds mingw-w64 alone and searches its include/
+// after mingw-w64's own headers, so every header mingw-w64 lacks came from
+// glibc: libstdc++ found <sys/single_threaded.h> and had shared_ptr read
+// __libc_single_threaded, which nothing on Windows defines, and every
+// program that used one failed to link (issue #43). CRAFTER_MINGW_DIR=/usr
+// did the same outside the Flatpak.
+//
+// The prefix staged here has the shape of such a /usr and needs no real
+// mingw-w64: glibc's features.h and <sys/single_threaded.h> in include/,
+// gcc's per-target directory, which clang derives libstdc++'s header
+// directory from, and a stand-in mingw-w64 tree whose libstdc++ has a std
+// module with nothing in it. The fixture is a static library, so nothing is
+// linked, and its C++ and C sources stop the compile when they see the
+// prefix's include/ or miss mingw-w64's or libstdc++'s headers.
+//
+// clang reaches such a prefix through a view of it (MakeMingwSysroot), which
+// has to be made where symbolic links can be, and a compile error in one of
+// its headers has to name the header where it is installed, not in the
+// view.
+
+extern "C" int setenv(const char* name, const char* value, int overwrite);
+
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+
+    void Write(const fs::path& file, std::string_view text) {
+        fs::create_directories(file.parent_path());
+        std::ofstream(file, std::ios::binary | std::ios::trunc) << text;
+    }
+
+    BuildResult BuildLibrary(const fs::path& project, std::string_view name, const fs::path& cFile) {
+        Configuration lib;
+        lib.path = project;
+        lib.name = std::string(name);
+        lib.outputName = std::string(name);
+        lib.target = "x86_64-w64-mingw32";
+        // Spelled out: the default, native, names the host's CPU, which is
+        // no x86 CPU on an aarch64 host.
+        lib.march = "x86-64";
+        lib.mtune = "x86-64";
+        lib.type = ConfigurationType::LibraryStatic;
+        std::array<fs::path, 0> ifaces = {};
+        std::array<fs::path, 1> impls = { fs::path(name) };
+        lib.GetInterfacesAndImplementations(ifaces, impls);
+        if (!cFile.empty()) lib.cFiles = { cFile };
+        std::unordered_map<fs::path, std::shared_future<BuildResult>> depResults;
+        std::mutex depMutex;
+        return Build(lib, depResults, depMutex);
+    }
+}
+
+int main() {
+    const fs::path scratch = fs::temp_directory_path() / "crafter-build-mingw-sysroot";
+    std::error_code ec;
+    fs::remove_all(scratch, ec);
+    fs::create_directories(scratch);
+    const fs::path project = scratch / "project";
+    fs::copy(fs::current_path() / "tests" / "MingwSysroot" / "fixture", project, fs::copy_options::recursive);
+
+    const fs::path prefix = scratch / "usr";
+    Write(prefix / "include" / "features.h", "#define __GLIBC__ 2\n");
+    Write(prefix / "include" / "sys" / "single_threaded.h", "extern char __libc_single_threaded;\n");
+    fs::create_directories(prefix / "lib" / "gcc" / "x86_64-w64-mingw32" / "15.1.0");
+    const fs::path mingw = prefix / "x86_64-w64-mingw32";
+    Write(mingw / "include" / "_mingw.h", "");
+    Write(mingw / "include" / "crafter_probe_error.h", "#error stopped in a mingw-w64 header\n");
+    Write(mingw / "include" / "c++" / "15.1.0" / "memory", "");
+    // Through std::format because the build's import scanner reads string
+    // literals as code: one holding "module std;" made this file a unit of a
+    // module std, and the test did not build.
+    Write(mingw / "include" / "c++" / "15.1.0" / "bits" / "std.cc", std::format("export module {};\n", "std"));
+    Write(mingw / "lib" / "libstdc++.modules.json", R"({"version": 1, "modules": [{"logical-name": "std", "source-path": "../include/c++/15.1.0/bits/std.cc", "is-std-library": true}]})");
+
+    {
+        // The view is made in the first place that can hold one. A cache on
+        // a filesystem that takes no symbolic links (vfat, exFAT, some CIFS
+        // and FUSE mounts) holds everything else crafter-build caches, so
+        // its failure must not be every mingw build's: this one can make
+        // nothing under mingw-sysroot, which is a file.
+        const fs::path blocked = scratch / "blocked";
+        Write(blocked / "mingw-sysroot", "");
+        const fs::path runtime = scratch / "runtime";
+        const std::array<fs::path, 2> parents = { blocked, runtime };
+        fs::path view;
+        try {
+            view = MakeMingwSysroot(prefix, parents);
+        } catch (const std::exception& e) {
+            Check(false, std::format("a view is made in the second place when the first can hold none: {}", e.what()));
+        }
+        if (!view.empty()) {
+            Check(view.string().starts_with(runtime.string()), std::format("the view is in the place that could hold it: {}", view.string()));
+            Check(fs::equivalent(view / "x86_64-w64-mingw32", mingw, ec), "the view shows the prefix's mingw-w64 tree");
+            Check(fs::equivalent(view / "lib" / "gcc", prefix / "lib" / "gcc", ec), "and gcc's directories");
+            Check(!fs::exists(view / "include") && !fs::exists(view / "lib" / "libstdc++.modules.json"), "and nothing else of the prefix's include/ and lib/");
+            fs::path again;
+            try {
+                again = MakeMingwSysroot(prefix, parents);
+            } catch (const std::exception& e) {
+                Check(false, std::format("a view already there is reused: {}", e.what()));
+            }
+            Check(again == view, "a view already there is reused");
+        }
+        const std::array<fs::path, 1> nowhere = { blocked };
+        std::string error;
+        try {
+            MakeMingwSysroot(prefix, nowhere);
+        } catch (const std::exception& e) {
+            error = e.what();
+        }
+        Check(error.contains(prefix.string()) && error.contains((blocked / "mingw-sysroot").string()), std::format("with no place for a view, the error names the prefix and why each place failed: {}", error));
+    }
+
+    // Before anything resolves the toolchain, which reads both once. The
+    // view and the std module cache go to the scratch directory too.
+    setenv("CRAFTER_MINGW_DIR", prefix.c_str(), 1);
+    setenv("XDG_CACHE_HOME", (scratch / "cache").c_str(), 1);
+
+    const BuildResult built = BuildLibrary(project, "probe", project / "cprobe");
+    Check(built.result.empty(), std::format("a Windows build sees mingw-w64's and libstdc++'s headers and not the prefix's own: {}", built.result));
+    Check(MingwSysroot() != prefix && MingwSysroot().string().starts_with((scratch / "cache").string()), std::format("a prefix holding a Linux system reaches clang through a view in the cache: {}", MingwSysroot().string()));
+
+    const BuildResult failed = BuildLibrary(project, "failing", {});
+    const std::string header = (mingw / "include" / "crafter_probe_error.h").string();
+    Check(failed.result.contains("stopped in a mingw-w64 header"), std::format("an error in a mingw-w64 header fails the build: {}", failed.result));
+    Check(failed.result.contains(header) && !failed.result.contains(MingwSysroot().string()), std::format("and names the header in the prefix, not in the view: {}", failed.result));
+
+    fs::remove_all(scratch, ec);
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}

tests/StdPcmRebuild/fixture/main.cpp

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+
+int main() {
+    std::println("rebuilt");
+}

tests/StdPcmRebuild/main.cpp

@@ -0,0 +1,85 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// A std module rebuilt in place has to recompile the units that import it.
+// The std PCM is rebuilt in its directory whenever its source changes under
+// the same compiler and flags: a libc++ update, or mingw-w64's under the
+// same prefix. A .cpp unit's depfile listed only its source and headers,
+// because the driver records imported BMIs for --precompile alone, so such
+// a unit kept its object and the next build only relinked it.
+//
+// The project is one main.cpp that imports std. Its object is dated before
+// std.pcm and after its source, as a std module rebuilt after the object
+// leaves them, and the next build has to compile it again. XDG_CACHE_HOME keeps the std PCM out of the real
+// cache, whose other users the change of date would rebuild.
+
+extern "C" int setenv(const char* name, const char* value, int overwrite);
+
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+
+    BuildResult BuildOnce(Configuration& cfg) {
+        std::unordered_map<fs::path, std::shared_future<BuildResult>> depResults;
+        std::mutex depMutex;
+        return Build(cfg, depResults, depMutex);
+    }
+}
+
+int main() {
+    const fs::path scratch = fs::temp_directory_path() / "crafter-build-std-pcm-rebuild";
+    std::error_code ec;
+    fs::remove_all(scratch, ec);
+    fs::create_directories(scratch);
+    const fs::path project = scratch / "project";
+    fs::copy(fs::current_path() / "tests" / "StdPcmRebuild" / "fixture", project, fs::copy_options::recursive);
+    setenv("XDG_CACHE_HOME", (scratch / "cache").c_str(), 1);
+
+    Configuration cfg;
+    cfg.path = project;
+    cfg.name = "rebuilt";
+    cfg.outputName = "rebuilt";
+    cfg.type = ConfigurationType::Executable;
+    std::array<fs::path, 0> ifaces = {};
+    std::array<fs::path, 1> impls = { "main" };
+    cfg.GetInterfacesAndImplementations(ifaces, impls);
+
+    BuildResult built = BuildOnce(cfg);
+    Check(built.result.empty(), std::format("the project builds: {}", built.result));
+    const fs::path object = cfg.BuildDir() / "main_impl.o";
+    const fs::path stdPcm = StdPcmDir(cfg) / "std.pcm";
+    Check(fs::exists(object) && fs::exists(stdPcm), std::format("{} and {} exist", object.string(), stdPcm.string()));
+    if (!fs::exists(object) || !fs::exists(stdPcm)) return 1;
+
+    const fs::file_time_type first = fs::last_write_time(object);
+    built = BuildOnce(cfg);
+    Check(built.result.empty(), std::format("an idle build succeeds: {}", built.result));
+    Check(fs::last_write_time(object) == first, "and compiles nothing again");
+
+    // The source before the object and the object before std.pcm, so that
+    // nothing but the std module makes the object stale.
+    const fs::file_time_type older = fs::last_write_time(stdPcm) - std::chrono::seconds(10);
+    fs::last_write_time(project / "main.cpp", older - std::chrono::seconds(10));
+    fs::last_write_time(object, older);
+    built = BuildOnce(cfg);
+    Check(built.result.empty(), std::format("the build after the std module succeeds: {}", built.result));
+    Check(fs::last_write_time(object) > older, "a unit importing a std module newer than its object is compiled again");
+
+    fs::remove_all(scratch, ec);
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}

tests/SysrootCompileFlags/main.cpp

@@ -19,6 +19,9 @@ using namespace Crafter;
 // sysroot's copy. It must not do so where there is no sysroot, for wasm (which
 // has its own sysroot handling) or for mingw (libstdc++, no libc++ at all).
 // The command is checked as text: no compiler runs.
+
+extern "C" int setenv(const char* name, const char* value, int overwrite);
+
 namespace {
     std::int32_t Failures = 0;
 
@@ -44,6 +47,15 @@ namespace {
 }
 
 int main() {
+    // A compile command names the std module's directory in the cache, and
+    // the mingw one can make the view of a system mingw prefix there
+    // (MingwSysroot), as with CRAFTER_MINGW_DIR=/usr or inside the Flatpak
+    // on a host with mingw-w64. Set before anything resolves the toolchain.
+    const fs::path scratch = fs::temp_directory_path() / "crafter-build-sysroot-compile-flags-cache";
+    std::error_code ec;
+    fs::remove_all(scratch, ec);
+    setenv("XDG_CACHE_HOME", scratch.c_str(), 1);
+
     const std::string sysroot = "/opt/crafter-build-test-sysroot";
     const std::string forced = std::format("-nostdinc++ -isystem {}/usr/include/c++/v1", sysroot);
 
@@ -57,6 +69,7 @@ int main() {
     Check(!CommandFor("wasm32-wasip1", sysroot).contains(forced), "wasm: left to its own sysroot handling");
     Check(!CommandFor("x86_64-w64-mingw32", sysroot).contains(forced), "mingw: libstdc++ target, libc++ headers are not forced");
 
+    fs::remove_all(scratch, ec);
     if (Failures > 0) {
         std::println(std::cerr, "{} assertions failed", Failures);
         return 1;

tests/ToolchainResolve/main.cpp

@@ -61,7 +61,7 @@ int main() {
         Check(tc.libcxxVersion == LibcxxVersion("/usr/include/c++/v1"), "the system libc++ headers state the release of the system module source");
         Check(tc.stdModulesDir.empty(), "no crafter-build prefix, no shipped module sources");
         Check(tc.wasiSysroot == "/usr/share/wasi-sysroot", "the system wasi sysroot is the default");
-        Check(tc.mingwSysroot.empty(), "outside a sandbox, clang finds mingw-w64 itself: no --sysroot");
+        Check(tc.mingwPrefix.empty(), "outside a sandbox, clang finds mingw-w64 itself: no prefix");
         Check(tc.LibcxxCompileFlags().empty() && tc.LibcxxLinkFlags().empty(), "the PATH fallback adds no libc++ flags");
     }
     {
@@ -144,7 +144,34 @@ int main() {
         in.env["CRAFTER_MINGW_DIR"] = "/opt/mingw";
         Toolchain tc = ResolveToolchain(in);
         Check(tc.wasiSysroot == "/opt/wasi", "CRAFTER_BUILD_WASI_SYSROOT overrides the wasi sysroot");
-        Check(tc.mingwSysroot == "/opt/mingw", "CRAFTER_MINGW_DIR names the mingw prefix passed as --sysroot");
+        Check(tc.mingwPrefix == "/opt/mingw", "CRAFTER_MINGW_DIR names the mingw prefix");
+        in.env["CRAFTER_MINGW_DIR"] = "mingw";
+        Check(ResolveToolchain(in).mingwPrefix == fs::absolute("mingw"), "a relative CRAFTER_MINGW_DIR is resolved against the working directory once, when the toolchain is");
+        // The prefix names directories, so one directory has one spelling:
+        // otherwise a trailing slash rebuilds every mingw build for nothing.
+        for (const char* spelling : {"/opt/mingw/", "/opt//mingw", "/opt/./mingw", "/opt/x/../mingw"}) {
+            in.env["CRAFTER_MINGW_DIR"] = spelling;
+            Check(ResolveToolchain(in).mingwPrefix == "/opt/mingw", std::format("{} is spelled /opt/mingw (got '{}')", spelling, ResolveToolchain(in).mingwPrefix.string()));
+        }
+    }
+    {
+        // A prefix that holds a Linux system's C library, as /usr does, is
+        // shown to clang through a view; one that holds mingw-w64 alone is
+        // the sysroot itself, Windows libraries in its include/ and lib/
+        // included.
+        const fs::path staged = fs::temp_directory_path() / "crafter-build-toolchain-mingw-prefixes";
+        fs::remove_all(staged);
+        Touch(staged / "system" / "include" / "features.h");
+        Touch(staged / "system" / "x86_64-w64-mingw32" / "include" / "_mingw.h");
+        Touch(staged / "dedicated" / "include" / "SDL2" / "SDL.h");
+        Touch(staged / "dedicated" / "x86_64-w64-mingw32" / "include" / "_mingw.h");
+        ToolchainInputs in;
+        in.env["CRAFTER_MINGW_DIR"] = (staged / "system").string();
+        Check(ResolveToolchain(in).mingwPrefixIsSystem, "a prefix with a C library's features.h holds a Linux system");
+        in.env["CRAFTER_MINGW_DIR"] = (staged / "dedicated").string();
+        Check(!ResolveToolchain(in).mingwPrefixIsSystem, "a prefix of mingw-w64 and Windows libraries does not");
+        Check(!ResolveToolchain({}).mingwPrefixIsSystem, "no prefix, nothing to show");
+        fs::remove_all(staged);
     }
     {
         // Sandboxed, the host's /usr stands in for the missing mingw-w64,
@@ -153,11 +180,12 @@ int main() {
         fs::remove_all(host);
         ToolchainInputs in;
         in.hostRoot = host;
-        Check(ResolveToolchain(in).mingwSysroot.empty(), "a host without mingw-w64 gives no --sysroot");
+        Check(ResolveToolchain(in).mingwPrefix.empty(), "a host without mingw-w64 gives no mingw prefix");
         fs::create_directories(host / "usr" / "x86_64-w64-mingw32");
-        Check(ResolveToolchain(in).mingwSysroot == host / "usr", "a host with mingw-w64 gives its /usr as the mingw prefix");
+        Check(ResolveToolchain(in).mingwPrefix == host / "usr", "a host with mingw-w64 gives its /usr as the mingw prefix");
+        Check(ResolveToolchain(in).mingwPrefixIsSystem, "the host's /usr is the host's system, with or without its C library's headers");
         in.env["CRAFTER_MINGW_DIR"] = "/opt/mingw";
-        Check(ResolveToolchain(in).mingwSysroot == "/opt/mingw", "CRAFTER_MINGW_DIR wins over the host's");
+        Check(ResolveToolchain(in).mingwPrefix == "/opt/mingw", "CRAFTER_MINGW_DIR wins over the host's");
         fs::remove_all(host);
     }
     {

tests/VariantId/main.cpp

@@ -160,6 +160,65 @@ int main() {
         Check(cfg.VariantId() == cfg.VariantId(GetToolchain()), "VariantId() is the variant under the running toolchain");
         Check(cfg.BuildDir().filename() == cfg.VariantId(GetToolchain()), "BuildDir is named for the running toolchain's variant");
     }
+    {
+        // The mingw-w64 prefix decides what a mingw build compiles against.
+        // Before issue #43 it was clang's sysroot as is, which put the host's
+        // headers on the include path, and with the prefix left out of the
+        // id, the release that fixed that reused the objects built against
+        // them: the next build only relinked, and failed to link as before.
+        Configuration mingw = MakeBase();
+        mingw.target = "x86_64-w64-mingw32";
+        Toolchain discovered;
+        discovered.clangxx = "/app/bin/clang++";
+        Toolchain hostUsr = discovered;
+        hostUsr.mingwPrefix = "/run/host/usr";
+        Toolchain optMingw = discovered;
+        optMingw.mingwPrefix = "/opt/mingw";
+        Check(mingw.VariantId(discovered) != mingw.VariantId(hostUsr), "a mingw prefix perturbs a mingw configuration's VariantId");
+        Check(mingw.VariantId(hostUsr) != mingw.VariantId(optMingw), "two mingw prefixes are two variants");
+        Configuration host = MakeBase();
+        Check(host.VariantId(discovered) == host.VariantId(hostUsr), "the mingw prefix leaves other targets' variants alone");
+
+        // The std module's directory is keyed the same way, and so is a
+        // prefix given whole against one reached through a view.
+        Check(StdPcmKey(mingw, discovered) != StdPcmKey(mingw, hostUsr), "a mingw prefix moves the mingw std module");
+        Check(StdPcmKey(mingw, hostUsr) != StdPcmKey(mingw, optMingw), "two mingw prefixes are two std modules");
+        Toolchain hostUsrWhole = hostUsr;
+        hostUsrWhole.mingwPrefixIsSystem = !hostUsr.mingwPrefixIsSystem;
+        Check(StdPcmKey(mingw, hostUsr) != StdPcmKey(mingw, hostUsrWhole), "a prefix given whole and one shown through a view are two std modules");
+        Check(mingw.VariantId(hostUsr) != mingw.VariantId(hostUsrWhole), "and two variants, through the std module");
+        Check(StdPcmKey(host, discovered) == StdPcmKey(host, hostUsr), "the mingw prefix leaves other targets' std modules alone");
+    }
+    {
+        // Everything the std module is compiled with keys its directory, not
+        // only target and march. -mtune was left out, so a project built
+        // with -mtune=generic shared one std PCM with project.cpp's
+        // -mtune=native, and the two rebuilt it in turn on every run, under
+        // every module built against it; two sysroots did the same.
+        Toolchain tc;
+        tc.clangxx = "/app/bin/clang++";
+        tc.clangVersion = "clang version 23.1.2";
+        Configuration native = MakeBase();
+        Configuration generic = MakeBase();
+        generic.mtune = "generic";
+        Check(StdPcmKey(native, tc) != StdPcmKey(generic, tc), "-mtune keys the std module");
+        Configuration sysrootA = MakeBase();
+        sysrootA.sysroot = "/opt/sysroot-a";
+        Configuration sysrootB = MakeBase();
+        sysrootB.sysroot = "/opt/sysroot-b";
+        Check(StdPcmKey(sysrootA, tc) != StdPcmKey(sysrootB, tc) && StdPcmKey(sysrootA, tc) != StdPcmKey(native, tc), "the sysroot keys the std module");
+        Configuration wasm = MakeBase();
+        wasm.target = "wasm32-wasip1";
+        wasm.sysroot = "/opt/wasi-sysroot";
+        Configuration relaxed = MakeBase();
+        relaxed.target = wasm.target;
+        relaxed.sysroot = wasm.sysroot;
+        relaxed.wasmVariantFlags = {"-mrelaxed-simd"};
+        Check(StdPcmKey(wasm, tc) != StdPcmKey(relaxed, tc), "a wasm variant pass's flags key the std module");
+        Check(StdPcmKey(native, tc) == StdPcmKey(MakeBase(), tc), "one configuration, one std module");
+        Check(StdPcmKey(native, tc).starts_with(std::format("{}-{}-{}-", native.target, native.march, tc.Key())), std::format("the directory still says target, march and toolchain (got '{}')", StdPcmKey(native, tc)));
+        Check(native.VariantId(tc) != generic.VariantId(tc) && sysrootA.VariantId(tc) != sysrootB.VariantId(tc), "a build moves with its std module");
+    }
 
     if (Failures > 0) {
         std::println(std::cerr, "{} assertions failed", Failures);