[clang] [Clang] Reject declaring an alias template with the same name as its template parameter. (PR #123533)

[Younan Zhang via cfe-commits]

================
@@ -13464,6 +13464,14 @@ Decl *Sema::ActOnAliasDeclaration(Scope *S, AccessSpecifier AS,
     }
     TemplateParameterList *TemplateParams = TemplateParamLists[0];
 
+    // Check shadowing of a template parameter name
+    for (NamedDecl *TP : TemplateParams->asArray()) {
+      if (NameInfo.getName() == TP->getDeclName()) {
+        DiagnoseTemplateParameterShadow(Name.StartLocation, TP);
+        return nullptr;
+      }
+    }
+
----------------
zyn0217 wrote:

Did you investigate why the if block around line 13428 doesn't work? I would have expected it to catch the issue earlier, before the creation of TypeAliasDecl.


```cpp
 // Warn about shadowing the name of a template parameter.
  if (Previous.isSingleResult() &&
      Previous.getFoundDecl()->isTemplateParameter()) {
    DiagnoseTemplateParameterShadow(Name.StartLocation,Previous.getFoundDecl());
    Previous.clear();
  }

```

https://github.com/llvm/llvm-project/pull/123533