Merge branch 'claude/issue-41': refuse a build that holds two builds of one module

PR #42, fixes #41. Build() now refuses a configuration whose transitive
dependencies hold two configurations exporting a module of the same
name, such as one project loaded along two paths with different arg
lists, before it builds anything, and names both paths. Until now both
builds ended up in the one program, which compiled cleanly and then
read garbage. Merged cleanly on top of claude/forward-project-args;
the suite, with claude/dependency-notes' fixtures, passes under the
new check.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/DuplicateModule/fixture/a/interfaces/A.cppm

@@ -0,0 +1,6 @@
+export module A;
+import std;
+import M;
+export std::int32_t ASize() {
+    return Size();
+}

tests/DuplicateModule/fixture/a/project.cpp

@@ -0,0 +1,24 @@
+// 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;
+
+// Loads M with the default, so M gets whatever A was given.
+extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> args) {
+    Configuration* m = LocalProject({ .projectFile = "../m/project.cpp" });
+
+    Configuration cfg;
+    cfg.path = "./";
+    cfg.name = "A";
+    cfg.outputName = "A";
+    cfg.type = ConfigurationType::LibraryStatic;
+    ApplyStandardArgs(cfg, args);
+    cfg.dependencies = { m };
+    std::array<fs::path, 1> ifaces = { "interfaces/A" };
+    std::array<fs::path, 0> impls = {};
+    cfg.GetInterfacesAndImplementations(ifaces, impls);
+    return cfg;
+}

tests/DuplicateModule/fixture/m/interfaces/M.cppm

@@ -0,0 +1,12 @@
+export module M;
+import std;
+// --wide gives S more members, so the two builds of M the test asks for
+// differ in what they declare.
+#ifdef WIDE
+export struct S { std::int32_t a; std::int32_t b; std::int64_t pad[4]; };
+#else
+export struct S { std::int32_t a; std::int32_t b; };
+#endif
+export std::int32_t Size() {
+    return static_cast<std::int32_t>(sizeof(S));
+}

tests/DuplicateModule/fixture/m/project.cpp

@@ -0,0 +1,22 @@
+// 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;
+
+// --wide is this project's own flag, and it changes what M declares.
+extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> args) {
+    Configuration cfg;
+    cfg.path = "./";
+    cfg.name = "M";
+    cfg.outputName = "M";
+    cfg.type = ConfigurationType::LibraryStatic;
+    ApplyStandardArgs(cfg, args);
+    if (std::ranges::contains(args, std::string_view("--wide"))) cfg.compileFlags.push_back("-DWIDE");
+    std::array<fs::path, 1> ifaces = { "interfaces/M" };
+    std::array<fs::path, 0> impls = {};
+    cfg.GetInterfacesAndImplementations(ifaces, impls);
+    return cfg;
+}

tests/DuplicateModule/fixture/p/main.cpp

@@ -0,0 +1,6 @@
+import std;
+import M;
+import A;
+int main() {
+    return Size() == ASize() ? 0 : 1;
+}

tests/DuplicateModule/fixture/p/project.cpp

@@ -0,0 +1,32 @@
+// 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;
+
+// Loads M itself and through A, which passes on all of its args. With
+// --split, the list P gives M leaves --wide out, as a filter meant for one
+// sibling would, and the two paths load M with two different lists.
+extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> args) {
+    const bool split = std::ranges::contains(args, std::string_view("--split"));
+    std::vector<std::string> mArgs;
+    for (std::string_view arg : args) {
+        if (!split || arg != "--wide") mArgs.emplace_back(arg);
+    }
+    Configuration* m = LocalProject({ .projectFile = "../m/project.cpp", .args = mArgs });
+    Configuration* a = LocalProject({ .projectFile = "../a/project.cpp" });
+
+    Configuration cfg;
+    cfg.path = "./";
+    cfg.name = "P";
+    cfg.outputName = "P";
+    cfg.type = ConfigurationType::Executable;
+    ApplyStandardArgs(cfg, args);
+    cfg.dependencies = { m, a };
+    std::array<fs::path, 0> ifaces = {};
+    std::array<fs::path, 1> impls = { "main" };
+    cfg.GetInterfacesAndImplementations(ifaces, impls);
+    return cfg;
+}

tests/DuplicateModule/main.cpp

@@ -0,0 +1,84 @@
+// 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 program can hold one module of each name, so Build() refuses a graph in
+// which two configurations export the same module: the same project loaded
+// along two paths with two arg lists, say. It used to build both and put
+// both on the consumer's module path. Clang took the first for every import
+// of the module, the ones inside the other build's consumers included, and
+// the program linked code compiled against two layouts of one struct: it
+// built cleanly, then read garbage or crashed (issue #41). The fixture is
+// P -> M and P -> A -> M. With --split, P gives M its args without --wide,
+// which changes what M declares, while A passes them on whole.
+namespace {
+    std::int32_t Failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++Failures;
+        }
+    }
+
+    BuildResult BuildPass(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 installedHome = GetCrafterBuildHome();
+    if (!fs::exists(installedHome / "Crafter.Build.cppm")) {
+        std::println(std::cerr, "SKIP: no module sources at {}", installedHome.string());
+        return 77;
+    }
+
+    // Staged because LoadProject and Build drop build/ and bin/ trees next
+    // to the project files.
+    const fs::path fixture = fs::current_path() / "tests" / "DuplicateModule" / "fixture";
+    const fs::path scratch = fs::temp_directory_path() / "crafter-build-duplicate-module";
+    std::error_code ec;
+    fs::remove_all(scratch, ec);
+    fs::create_directories(scratch);
+    fs::copy(fixture, scratch, fs::copy_options::recursive);
+
+    // From P's own directory, as crafter-build runs it: the fixture's
+    // LocalProject paths are relative to the working directory.
+    const fs::path cwd = fs::current_path();
+    fs::current_path(scratch / "p");
+    try {
+        // Two lists for M: refused before anything is built.
+        {
+            std::array<std::string_view, 2> args = { "--wide", "--split" };
+            Configuration cfg = LoadProject("project.cpp", args);
+            const BuildResult built = BuildPass(cfg);
+            Check(!built.result.empty(), "a graph with two builds of one module fails to build");
+            Check(built.result.contains("module M is built twice for P"), std::format("the error names the module and the program (got: {})", built.result));
+            Check(built.result.contains("P -> M (") && built.result.contains("P -> A -> M ("), std::format("the error names the path to each build (got: {})", built.result));
+            Check(!fs::exists(scratch / "m" / "bin") && !fs::exists(scratch / "a" / "bin") && !fs::exists(scratch / "p" / "bin"), "nothing was built");
+        }
+
+        // One list along both paths: one M, and the build goes through.
+        {
+            std::array<std::string_view, 1> args = { "--wide" };
+            Configuration cfg = LoadProject("project.cpp", args);
+            const bool oneM = cfg.dependencies.size() == 2 && cfg.dependencies[1]->dependencies.size() == 1 && cfg.dependencies[0] == cfg.dependencies[1]->dependencies[0];
+            Check(oneM, "the same list along both paths loads one M");
+            const BuildResult built = BuildPass(cfg);
+            Check(built.result.empty(), std::format("the graph with one M builds (got: {})", built.result));
+        }
+    } catch (const std::exception& e) {
+        fs::current_path(cwd);
+        std::println(std::cerr, "FAIL: threw: {}", e.what());
+        return 1;
+    }
+    fs::current_path(cwd);
+
+    return Failures > 0 ? 1 : 0;
+}