[clang] [clang][modules] Minor improvements to diagnosing `out of date` errors (PR #136612)

[Jan Svoboda via cfe-commits]

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@@ -3349,10 +3351,29 @@ ASTReader::ReadControlBlock(ModuleFile &F,
                                       .getModuleCache()
                                       .getInMemoryModuleCache()
                                       .isPCMFinal(F.FileName);
-      if (isDiagnosedResult(Result, Capabilities) || recompilingFinalized)
+      if (isDiagnosedResult(Result, Capabilities) || recompilingFinalized) {
         Diag(diag::note_module_file_imported_by)
             << F.FileName << !F.ModuleName.empty() << F.ModuleName;
 
+        // When a Filemap is used, the ImportedFile may be different than what
+        // was encoded as a dependency in StoredFile.
+        // When this happens, look up both files when reporting that they
+        // differ. If either file does not exist, fall back to a string path
+        // check.
+        if (UseFilemap && !F.ModuleName.empty() && !StoredFile.empty()) {
+          auto ImportedFileRef =
+              PP.getFileManager().getOptionalFileRef(ImportedFile);
+          auto StoredFileRef =
+              PP.getFileManager().getOptionalFileRef(StoredFile);
+          const bool MissingFiles = !ImportedFileRef || !StoredFileRef;
+          if ((MissingFiles && (StoredFile != ImportedFile)) ||
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jansvoboda11 wrote:

> `StoredFile` is never checked for existence anywhere else

That makes sense.

> & this code path can only happen if `ImportedFile` existed at the time of `lookupModuleFile`.

Then `FileManager` will never report it as missing after that point, unless I'm missing something.

> It would be nice to reuse the FileEntry created by the ModuleManager, but it goes out of scope by the time this check happens.

Does it? It seems like it gets stored as `ModuleFile::File`.


https://github.com/llvm/llvm-project/pull/136612