[clang] [clang] Support header shadowing diagnostics in Clang header search (PR #162491)

[Tom Honermann via cfe-commits]

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@@ -881,6 +886,35 @@ diagnoseFrameworkInclude(DiagnosticsEngine &Diags, SourceLocation IncludeLoc,
         << IncludeFilename;
 }
 
+/// Return true if a shadow has been detected and the caller should
+/// stop and return the first-found file and module, false otherwise.
+static bool checkAndStoreCandidate(
+    ModuleMap::KnownHeader *SuggestedModule, OptionalFileEntryRef CandidateFile,
+    StringRef CandidateDir, DiagnosticsEngine &Diags, StringRef Filename,
+    SourceLocation IncludeLoc, ModuleMap::KnownHeader &FirstModule,
+    OptionalFileEntryRef &FirstHeader, SmallString<1024> &FirstDir) {
+  if (!FirstHeader) {
+    // Found the first candidate
+    FirstHeader = CandidateFile;
+    FirstDir = CandidateDir;
+    if (SuggestedModule)
+      FirstModule = *SuggestedModule;
+    return false;
+  }
+
+  if (FirstDir != CandidateDir) {
+    // Found a second candidate from a different directory
+    Diags.Report(IncludeLoc, diag::warn_header_shadowed)
+        << Filename << FirstDir << CandidateDir;
+    if (SuggestedModule)
+      *SuggestedModule = FirstModule;
+    return true;
+  }
+
+  // Found a candidate from the same directory as the first one
+  return false;
----------------
tahonermann wrote:

Clang elides duplicate search paths during search path ordering. I think finding the same header file in the same location can only happen in some cases where the same include path is present in both the quoted search path and angle search path (and that might only be in existing cases where Clang doesn't match GCC behavior). It could also happen with the corrected MSVC search path ordering from [my draft PR](https://github.com/llvm/llvm-project/pull/105738), but since MSVC doesn't support `#include_next`, it doesn't really matter.

https://github.com/llvm/llvm-project/pull/162491