[clang] Do not pass -canonical-system-headers on Windows by default

[aeubanks]

Canonicalizing paths on Windows leads to unexpected things like changing
drive letters. As a short term fix, do not canonicalize system headers
on Windows by default.

Fixes #70011

[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-driver

Author: Arthur Eubanks (aeubanks)

Changes

Canonicalizing paths on Windows leads to unexpected things like changing
drive letters. As a short term fix, do not canonicalize system headers
on Windows by default.

Fixes #70011

---

Full diff: https://github.com/llvm/llvm-project/pull/71097.diff

3 Files Affected:

• (modified) clang/lib/Driver/ToolChains/Clang.cpp (+12-1)
• (added) clang/test/Driver/canonical-system-headers-win.c (+9)
• (modified) clang/test/Driver/canonical-system-headers.c (+3)

diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 79f7fba22570746..b94728f79e0a175 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -1180,8 +1180,19 @@ void Clang::AddPreprocessingOptions(Compilation &C, const JobAction &JA,
     if (ArgM->getOption().matches(options::OPT_M) ||
         ArgM->getOption().matches(options::OPT_MD))
       CmdArgs.push_back("-sys-header-deps");
+
+      // #70011: Canonicalization on Windows does unexpected things like change
+      // drive letters.
+      // FIXME: find and use Windows API that canonicalizes paths except for
+      // drive letter.
+#if defined(_WIN32) || defined(_WIN64)
+    bool CanonicalSystemHeadersDefault = false;
+#else
+    bool CanonicalSystemHeadersDefault = true;
+#endif
     if (Args.hasFlag(options::OPT_canonical_prefixes,
-                     options::OPT_no_canonical_prefixes, true))
+                     options::OPT_no_canonical_prefixes,
+                     CanonicalSystemHeadersDefault))
       CmdArgs.push_back("-canonical-system-headers");
     if ((isa<PrecompileJobAction>(JA) &&
          !Args.hasArg(options::OPT_fno_module_file_deps)) ||
diff --git a/clang/test/Driver/canonical-system-headers-win.c b/clang/test/Driver/canonical-system-headers-win.c
new file mode 100644
index 000000000000000..cc3ec57ff4de614
--- /dev/null
+++ b/clang/test/Driver/canonical-system-headers-win.c
@@ -0,0 +1,9 @@
+// REQUIRES: system-windows
+// The default on Windows is false due to #70011.
+
+// RUN: %clang -MD -no-canonical-prefixes -### %s 2>&1 | FileCheck %s --check-prefix=CHECK-NO
+// RUN: %clang -MD -canonical-prefixes -### %s 2>&1 | FileCheck %s --check-prefix=CHECK-YES
+// RUN: %clang -MD -### %s 2>&1 | FileCheck %s --check-prefix=CHECK-NO
+
+// CHECK-YES: "-canonical-system-headers"
+// CHECK-NO-NOT: "-canonical-system-headers"
diff --git a/clang/test/Driver/canonical-system-headers.c b/clang/test/Driver/canonical-system-headers.c
index a7ab57521fc2249..d37b5925c73035d 100644
--- a/clang/test/Driver/canonical-system-headers.c
+++ b/clang/test/Driver/canonical-system-headers.c
@@ -1,3 +1,6 @@
+// REQUIRES: !system-windows
+// The default on Windows is false due to #70011.
+
 // RUN: %clang -MD -no-canonical-prefixes -### %s 2>&1 | FileCheck %s --check-prefix=CHECK-NO
 // RUN: %clang -MD -canonical-prefixes -### %s 2>&1 | FileCheck %s --check-prefix=CHECK-YES
 // RUN: %clang -MD -### %s 2>&1 | FileCheck %s --check-prefix=CHECK-YES

[MaskRay]

_WIN32 is defined on 64-bit Windows.

[MaskRay]

UNSUPPORTED: system-windows

[aeubanks]

closing in favor of #71697