[clang] nolock/noalloc attributes (PR #84983)

[via cfe-commits]

================
@@ -3922,6 +3922,42 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD, Scope *S,
     return true;
   }
 
+  const auto OldFX = Old->getFunctionEffects();
+  const auto NewFX = New->getFunctionEffects();
+  if (OldFX != NewFX) {
+    const auto Diffs = FunctionEffectSet::differences(OldFX, NewFX);
+    for (const auto &Item : Diffs) {
+      const FunctionEffect *Effect = Item.first;
+      const bool Adding = Item.second;
+      if (Effect->diagnoseRedeclaration(Adding, *Old, OldFX, *New, NewFX)) {
+        Diag(New->getLocation(),
+             diag::warn_mismatched_func_effect_redeclaration)
+            << Effect->name();
+        Diag(Old->getLocation(), diag::note_previous_declaration);
+      }
+    }
+
+    const auto MergedFX = OldFX | NewFX;
+
+    // Having diagnosed any problems, prevent further errors by applying the
+    // merged set of effects to both declarations.
+    auto applyMergedFX = [&](FunctionDecl *FD) {
+      const auto *FPT = FD->getType()->getAs<FunctionProtoType>();
+      FunctionProtoType::ExtProtoInfo EPI = FPT->getExtProtoInfo();
+      EPI.FunctionEffects = MergedFX;
+      QualType ModQT = Context.getFunctionType(FD->getReturnType(),
+                                               FPT->getParamTypes(), EPI);
+
+      FD->setType(ModQT);
+    };
+
+    applyMergedFX(Old);
+    applyMergedFX(New);
+
+    OldQType = Old->getType();
----------------
Sirraide wrote:

> I think that `foo()` ought to get verified because by the time verification happens, the redeclaration has been seen.

I agree that that would make sense. 

> `MergeFunctionDecl` is unhappy that `New` has a different canonical type.

Iirc we’re adjusting the type(s) here for the purposes of the comparison only (there’s a `NewQTypeForComparison` or whatever it’s called); you may have to change either that (e.g. always drop the noalloc/nolock attributes from the new type or add them if only the old type has them) or possibly even do the same for the `OldQType` (i.e. introduce an `OldQTypeForComparison` and modify that one if necessary); that’s fine because those types are only for comparing the function types (as the name would suggest); the actual type of the declarations isn’t changed.

https://github.com/llvm/llvm-project/pull/84983