[clang] [Clang] Emit error for duplicate mangled names within a lambda (PR #107581)

[Sander de Smalen via cfe-commits]

================
@@ -4652,18 +4652,35 @@ llvm::Constant *CodeGenModule::GetOrCreateLLVMFunction(
 
     // If there are two attempts to define the same mangled name, issue an
     // error.
-    if (IsForDefinition && !Entry->isDeclaration()) {
-      GlobalDecl OtherGD;
-      // Check that GD is not yet in DiagnosedConflictingDefinitions is required
-      // to make sure that we issue an error only once.
-      if (lookupRepresentativeDecl(MangledName, OtherGD) &&
-          (GD.getCanonicalDecl().getDecl() !=
-           OtherGD.getCanonicalDecl().getDecl()) &&
-          DiagnosedConflictingDefinitions.insert(GD).second) {
+    GlobalDecl OtherGD;
+    // Check that GD is not yet in DiagnosedConflictingDefinitions is required
+    // to make sure that we issue an error only once.
+    if (GD && lookupRepresentativeDecl(MangledName, OtherGD) &&
+        (GD.getCanonicalDecl().getDecl() !=
+         OtherGD.getCanonicalDecl().getDecl()) &&
+        DiagnosedConflictingDefinitions.insert(GD).second) {
+      if (IsForDefinition && !Entry->isDeclaration()) {
         getDiags().Report(D->getLocation(), diag::err_duplicate_mangled_name)
             << MangledName;
         getDiags().Report(OtherGD.getDecl()->getLocation(),
                           diag::note_previous_definition);
+      } else {
+        // For lambdas, it's possible to create the same mangled name from
+        // different function prototypes. For example, two FPTs may have
+        // identical types but incompatible function attributes which we should
+        // not allow.
+        auto *MD = dyn_cast<CXXMethodDecl>(D);
+        if (MD && MD->getParent()->isLambda()) {
+          const FunctionDecl *OtherFD =
+              cast_or_null<FunctionDecl>(OtherGD.getDecl());
+          if (FD && FD->hasPrototype() && OtherFD && OtherFD->hasPrototype()) {
+            if (FD->getType()->getAs<FunctionProtoType>() !=
+                OtherFD->getType()->getAs<FunctionProtoType>())
----------------
sdesmalen-arm wrote:

I don't understand why there's a need to check the `FunctionProtoType`s here, because the check `GD.getCanonicalDecl().getDecl() != OtherGD.getCanonicalDecl().getDecl()` has already checked if the declarations and types are the same.

Note that I tried a smaller change like this:
```
bool IsLambdaInstantiation =
    D && isa<CXXMethodDecl>(D) &&     
    cast<CXXMethodDecl>(D)->getParent()->isLambda();
if (IsForDefinition && (!Entry->isDeclaration() || IsLambdaInstantiation)) {
```

And got the desired result. Would the above be sufficient or is there something this is not covering?

https://github.com/llvm/llvm-project/pull/107581