[clang] [clang] Support --sysroot= for ${arch}-windows-msvc targets (PR #96417)

[via cfe-commits]

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@@ -763,19 +789,36 @@ void MSVCToolChain::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
   // As a fallback, select default install paths.
   // FIXME: Don't guess drives and paths like this on Windows.
   const StringRef Paths[] = {
-    "C:/Program Files/Microsoft Visual Studio 10.0/VC/include",
-    "C:/Program Files/Microsoft Visual Studio 9.0/VC/include",
-    "C:/Program Files/Microsoft Visual Studio 9.0/VC/PlatformSDK/Include",
-    "C:/Program Files/Microsoft Visual Studio 8/VC/include",
-    "C:/Program Files/Microsoft Visual Studio 8/VC/PlatformSDK/Include"
-  };
+      "C:/Program Files/Microsoft Visual Studio 10.0/VC/include",
+      "C:/Program Files/Microsoft Visual Studio 9.0/VC/include",
+      "C:/Program Files/Microsoft Visual Studio 9.0/VC/PlatformSDK/Include",
+      "C:/Program Files/Microsoft Visual Studio 8/VC/include",
+      "C:/Program Files/Microsoft Visual Studio 8/VC/PlatformSDK/Include"};
   addSystemIncludes(DriverArgs, CC1Args, Paths);
 #endif
 }
 
 void MSVCToolChain::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
                                                  ArgStringList &CC1Args) const {
   // FIXME: There should probably be logic here to find libc++ on Windows.
+  if (DriverArgs.hasArg(options::OPT_nostdinc, options::OPT_nostdlibinc,
+                        options::OPT_nostdincxx))
+    return;
+  if (getDriver().SysRoot.empty())
+    return;
+  switch (GetCXXStdlibType(DriverArgs)) {
+  case ToolChain::CST_Stl:
+    addStlIncludePaths(DriverArgs, CC1Args);
+    break;
+  case ToolChain::CST_Libstdcxx:
+    addLibStdCXXIncludePaths(DriverArgs, CC1Args);
+    break;
+  case ToolChain::CST_Libcxx:
+    addLibCxxIncludePaths(DriverArgs, CC1Args);
+    break;
+  default:
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zmodem wrote:

> The problem is that the warnings are too much.

Can you elaborate?

The coding standards specifically call out not using default for switches covering enums: https://llvm.org/docs/CodingStandards.html#don-t-use-default-labels-in-fully-covered-switches-over-enumerations

https://github.com/llvm/llvm-project/pull/96417