fix: build std.compat with Clang and MSVC STL

src/build/prepare/toolchain_env.cpp

@@ -231,21 +231,22 @@ bind_msvc_sysroot(mcpp::toolchain::Toolchain& tc,
     // by its own search. A toolset without one leaves `import std` unavailable
     // rather than borrowing another toolset's.
     //
-    // Only `std` is rebound. Detection never gave this row a `std.compat`
-    // source, and the clang builder for it passes the file without
-    // `-x c++-module`: given `std.compat.ixx`, clang takes it for linker
-    // input, `--precompile` writes nothing and exits 0, and the next command
-    // fails on the missing BMI (measured on the Windows runners).
+    // Bind both standard modules to this toolset. A compat module from another
+    // STL cannot safely import the selected std BMI.
     std::error_code ec;
     const auto ixx = choice->toolsDir / "modules" / "std.ixx";
     const bool msvcStl = tc.stdModuleSource.empty()
                       || tc.stdModuleSource.filename() == "std.ixx";
     if (msvcStl && std::filesystem::exists(ixx, ec)) {
         tc.stdModuleSource   = ixx;
+        const auto compat = ixx.parent_path() / "std.compat.ixx";
+        tc.stdCompatSource = std::filesystem::exists(compat, ec)
+            ? compat : std::filesystem::path{};
         tc.hasImportStd      = true;
         tc.importStdMinLevel = msvc::std_module_min_level_for_stl(ixx);
     } else if (msvcStl && !tc.stdModuleSource.empty()) {
         tc.stdModuleSource.clear();
+        tc.stdCompatSource.clear();
         tc.hasImportStd = false;
     }
 

src/toolchain/clang.cppm

@@ -566,8 +566,13 @@ std::vector<std::string> std_compat_build_commands(const Toolchain& tc,
     auto absBmi    = (cacheDir / relBmi).string();
     auto absStdBmi = (cacheDir / relStdBmi).string();
     auto absObj    = (cacheDir / "std.compat.o").string();
+    // MSVC STL ships .ixx, which Clang does not infer as C++ module input.
+    // Apply the override only when parsing source, never when compiling a BMI.
+    std::string ixxFlags = (tc.stdCompatSource.extension() == ".ixx")
+        ? " -x c++-module -Wno-include-angled-in-module-purview"
+        : "";
     return {
-        std::format("{}{} {}{} -Wno-reserved-module-identifier{}{} "
+        std::format("{}{} {}{} -Wno-reserved-module-identifier{}{}{} "
                     "-fmodule-file=std={} "
                     "--precompile {} -o {} 2>&1",
                     mcpp::toolchain::compiler_env_prefix(tc),
@@ -576,6 +581,7 @@ std::vector<std::string> std_compat_build_commands(const Toolchain& tc,
                     crtToken,
                     sysrootFlag,
                     precompileFlags,
+                    ixxFlags,
                     absStdBmi,
                     mcpp::xlings::shq(tc.stdCompatSource.string()),
                     mcpp::xlings::shq(absBmi)),

tests/unit/test_toolchain_stdmod.cpp

@@ -71,6 +71,27 @@ TEST(ToolchainStdmod, ClangStdCompatCommandsUseRequestedStandard) {
     }
 }
 
+TEST(ToolchainStdmod, MsvcCompatSourceIsParsedAsAModuleButItsBmiIsNot) {
+    auto tc = clang_toolchain();
+    tc.stdModuleSource = "std.ixx";
+    tc.stdCompatSource = "std.compat.ixx";
+    auto cmds = clang::std_compat_build_commands(
+        tc, "cache", "cache/pcm.cache/std.compat.pcm",
+        "cache/pcm.cache/std.pcm", "", "-std=c++23", "-fms-runtime-lib=dll");
+    ASSERT_EQ(cmds.size(), 2u);
+    EXPECT_NE(cmds[0].find("-x c++-module"), std::string::npos);
+    EXPECT_EQ(cmds[1].find("-x c++-module"), std::string::npos);
+    for (auto const& cmd : cmds) {
+        EXPECT_NE(cmd.find("-fmodule-file=std="), std::string::npos);
+        EXPECT_NE(cmd.find("-fms-runtime-lib=dll"), std::string::npos);
+    }
+    tc.stdCompatSource = "std.compat.cppm";
+    cmds = clang::std_compat_build_commands(
+        tc, "cache", "cache/pcm.cache/std.compat.pcm",
+        "cache/pcm.cache/std.pcm", "", "-std=c++23");
+    EXPECT_EQ(cmds[0].find("-x c++-module"), std::string::npos);
+}
+
 // THE PRECOMPILE HAS TO KNOW WHICH MACHINE, AND ONLY ONE OF TWO SOURCES EVER
 // CARRIES IT.
 //