[clang] [clang][Modules] Diagnosing Module Redefinition Across ModuleMaps (PR #190085)

[Cyndy Ishida via cfe-commits]

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@@ -1756,9 +1756,24 @@ void ModuleMapLoader::handleModuleDecl(const modulemap::ModuleDecl &MD) {
   if (Module *Existing = Map.lookupModuleQualified(ModuleName, ActiveModule)) {
     // We might see a (re)definition of a module that we already have a
     // definition for in four cases:
-    //  - If we loaded one definition from an AST file and we've just found a
-    //    corresponding definition in a module map file, or
+    //  - If we loaded one definition from an AST file and we've just found the
+    //    corresponding definition in the same module map file, or
     bool LoadedFromASTFile = Existing->IsFromModuleFile;
+    if (LoadedFromASTFile) {
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cyndyishida wrote:

IIUC, this isn't actually checking module decls. It's comparing FileEntries as a proxy. What would happen if the compiler encountered 1 modulemap that had conflicting module declarations for the same module name? This is a contrived example, but unless it's caught somewhere else, I think this code would not diagnose that. Could you check for a more precise comparison? Maybe the `DefinitionLoc`, if not the explicit Definition? 

https://github.com/llvm/llvm-project/pull/190085