Index: clang/include/clang/Driver/Options.td =================================================================== --- clang/include/clang/Driver/Options.td +++ clang/include/clang/Driver/Options.td @@ -4737,6 +4737,8 @@ def _SLASH_Tp : CLCompileJoinedOrSeparate<"Tp">, HelpText<"Treat as C++ source file">, MetaVarName<"">; def _SLASH_TP : CLCompileFlag<"TP">, HelpText<"Treat all source files as C++">; +def _SLASH_vctoolsdir : CLJoinedOrSeparate<"vctoolsdir">, + HelpText<"Path to the VCToolChain">, MetaVarName<"">; def _SLASH_volatile_iso : Option<["/", "-"], "volatile:iso", KIND_FLAG>, Group<_SLASH_volatile_Group>, Flags<[CLOption, DriverOption]>, HelpText<"Volatile loads and stores have standard semantics">; Index: clang/lib/Driver/ToolChains/MSVC.cpp =================================================================== --- clang/lib/Driver/ToolChains/MSVC.cpp +++ clang/lib/Driver/ToolChains/MSVC.cpp @@ -66,6 +66,19 @@ static bool getSystemRegistryString(const char *keyPath, const char *valueName, std::string &value, std::string *phValue); +// Check command line arguments to try and find a toolchain. +static bool +findVCToolChainViaCommandLine(const ArgList &Args, std::string &Path, + MSVCToolChain::ToolsetLayout &VSLayout) { + // Trust the value supplied by the user. + if (Arg *A = Args.getLastArg(options::OPT__SLASH_vctoolsdir)) { + Path = A->getValue(); + VSLayout = MSVCToolChain::ToolsetLayout::VS2017OrNewer; + return true; + } + return false; +} + // Check various environment variables to try and find a toolchain. static bool findVCToolChainViaEnvironment(std::string &Path, MSVCToolChain::ToolsetLayout &VSLayout) { @@ -751,7 +764,8 @@ // what they want to use. // Failing that, just try to find the newest Visual Studio version we can // and use its default VC toolchain. - findVCToolChainViaEnvironment(VCToolChainPath, VSLayout) || + findVCToolChainViaCommandLine(Args, VCToolChainPath, VSLayout) || + findVCToolChainViaEnvironment(VCToolChainPath, VSLayout) || findVCToolChainViaSetupConfig(VCToolChainPath, VSLayout) || findVCToolChainViaRegistry(VCToolChainPath, VSLayout); } Index: clang/test/Driver/cl-options.c =================================================================== --- clang/test/Driver/cl-options.c +++ clang/test/Driver/cl-options.c @@ -682,4 +682,7 @@ // CLANG-NOT: "--dependent-lib=libcmt" // CLANG-NOT: "-vectorize-slp" +// vctoolsdir is handled by the driver; just check that we don't error. Pass -c because fakedir isn't a real toolchain path +// RUN: %clang_cl -c -vctoolsdir fakedir -- %s 2>&1 + void f() { }