[Clang][Sema] Reject declaring an alias template with the same name as its template parameter.

clang/docs/ReleaseNotes.rst

@@ -956,6 +956,7 @@ Bug Fixes to C++ Support
 - Fixed a crash caused by the incorrect construction of template arguments for CTAD alias guides when type
   constraints are applied. (#GH122134)
 - Fixed canonicalization of pack indexing types - Clang did not always recognized identical pack indexing. (#GH123033)
+- Clang now rejects declaring an alias template with the same name as its template parameter. (#GH123423)
 
 
 Bug Fixes to AST Handling

clang/lib/Sema/SemaDeclCXX.cpp

@@ -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;
+      }
+    }
+
     // Check that we can declare a template here.
     if (CheckTemplateDeclScope(S, TemplateParams))
       return nullptr;

clang/test/CXX/temp/temp.decls/temp.variadic/fixed-expansion.cpp

@@ -121,8 +121,8 @@ namespace PartialSpecialization {
 
 namespace FixedAliasTemplate {
   template<typename,typename,typename> struct S {};
-  template<typename T, typename U> using U = S<T, int, U>; // expected-note 2{{template parameter is declared here}}
-  template<typename...Ts> U<Ts...> &f(U<Ts...>, Ts...); // expected-error 2{{pack expansion used as argument for non-pack parameter of alias template}}
+  template<typename T, typename U> using Z = S<T, int, U>; // expected-note 2{{template parameter is declared here}}
+  template<typename...Ts> Z<Ts...> &f(Z<Ts...>, Ts...); // expected-error 2{{pack expansion used as argument for non-pack parameter of alias template}}
   S<int, int, double> &s1 = f({}, 0, 0.0); // expected-error {{no matching function}}
 }
 

clang/test/SemaCXX/alias-template.cpp

@@ -65,7 +65,8 @@ namespace InFunctions {
   template<typename...T> struct S3 { // expected-note {{template parameter is declared here}}
     template<typename Z> using T = int; // expected-error {{declaration of 'T' shadows template parameter}}
   };
-  template<typename Z> using Z = Z;
+  template<typename Z> // expected-note {{template parameter is declared here}}
+  using Z = Z; // expected-error {{declaration of 'Z' shadows template parameter}}
 }
 
 namespace ClassNameRedecl {
@@ -191,4 +192,4 @@ int g = sfinae_me<int>(); // expected-error{{no matching function for call to 's
 
 namespace NullExceptionDecl {
 template<int... I> auto get = []() { try { } catch(...) {}; return I; }; // expected-error{{initializer contains unexpanded parameter pack 'I'}}
-}
+}