[clang] [Clang] Only compare template params of potential overload after checking their decl context (PR #78139)

[Timm Baeder via cfe-commits]

================
@@ -1259,6 +1259,40 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
   if ((OldTemplate == nullptr) != (NewTemplate == nullptr))
     return true;
 
+  // Is the function New an overload of the function Old?
+  QualType OldQType = SemaRef.Context.getCanonicalType(Old->getType());
+  QualType NewQType = SemaRef.Context.getCanonicalType(New->getType());
+
+  // Compare the signatures (C++ 1.3.10) of the two functions to
+  // determine whether they are overloads. If we find any mismatch
+  // in the signature, they are overloads.
+
+  // If either of these functions is a K&R-style function (no
+  // prototype), then we consider them to have matching signatures.
+  if (isa<FunctionNoProtoType>(OldQType.getTypePtr()) ||
+      isa<FunctionNoProtoType>(NewQType.getTypePtr()))
+    return false;
+
+  const FunctionProtoType *OldType = cast<FunctionProtoType>(OldQType);
+  const FunctionProtoType *NewType = cast<FunctionProtoType>(NewQType);
----------------
tbaederr wrote:

```suggestion
  const auto *OldType = cast<FunctionProtoType>(OldQType);
  const auto *NewType = cast<FunctionProtoType>(NewQType);
```

https://github.com/llvm/llvm-project/pull/78139