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

[Valentyn Yukhymenko 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;
+      }
+    }
+
----------------
BaLiKfromUA wrote:

Thank you for the review!

I also was with this expectation and made some investigation (but be aware that it might not be 100% correct 😛).

According to my debugging, the reason for `LookupName(Previous, S);` at line `13425` not seeing template params of alias template is logic on the beginning of a function (line `13401`):
```
S = S->getDeclParent();
```

If you remove this line, the behavior mentioned in the issue is successfully diagnosed.

Initially, I thought that this logic was important for some other template aliasing functionality that I was missing, but I run locally all clang tests without this line, and seems like it doesn't.

So I am probably overthinking and we could just remove line `13401` instead of implementing this loop.

@zyn0217 Let me know if I am missing something! Otherwise, I will send a fix without this loop later today.



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