feat: robust MSVC STL discovery via msvc.cppm (fixes std.ixx not found)#54
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New module mcpp.toolchain.msvc with three discovery strategies: 1. vswhere.exe (Microsoft's official VS locator) — most reliable 2. Environment variables (VSINSTALLDIR, VS*COMNTOOLS) 3. Well-known paths (VS 2017-2025, all editions + BuildTools) Replaces the hardcoded VS2022 path in clang.cppm with msvc::find_std_module_source(). Fixes "no std module source" error on machines with non-standard VS installations. Also provides find_cl() for future MSVC toolchain support.
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Summary
Fixes
error: source imports std but toolchain 'clang 20.1.7' provides no std module sourceon Windows machines where Visual Studio is not installed at the hardcoded CI path.Root cause:
clang.cppmhardcodedC:\Program Files\Microsoft Visual Studio\2022to findstd.ixx. This fails on machines with VS2019, VS installed in non-default paths, or Build Tools only.Fix: New
src/toolchain/msvc.cppmmodule with three discovery strategies:VSINSTALLDIR,VS170COMNTOOLS, etc.clang.cppmnow callsmsvc::find_std_module_source()instead of hardcoded paths.Test plan