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

clang/docs/ReleaseNotes.rst

@@ -990,6 +990,7 @@ Bug Fixes to C++ Support
 - Fix immediate escalation not propagating through inherited constructors.  (#GH112677)
 - Fixed assertions or false compiler diagnostics in the case of C++ modules for
   lambda functions or inline friend functions defined inside templates (#GH122493).
+- 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

@@ -13406,8 +13406,6 @@ Decl *Sema::ActOnAliasDeclaration(Scope *S, AccessSpecifier AS,
                                   SourceLocation UsingLoc, UnqualifiedId &Name,
                                   const ParsedAttributesView &AttrList,
                                   TypeResult Type, Decl *DeclFromDeclSpec) {
-  // Get the innermost enclosing declaration scope.
-  S = S->getDeclParent();
 
   if (Type.isInvalid())
     return nullptr;
@@ -13458,6 +13456,9 @@ Decl *Sema::ActOnAliasDeclaration(Scope *S, AccessSpecifier AS,
   CheckTypedefForVariablyModifiedType(S, NewTD);
   Invalid |= NewTD->isInvalidDecl();
 
+  // Get the innermost enclosing declaration scope.
+  S = S->getDeclParent();
+
   bool Redeclaration = false;
 
   NamedDecl *NewND;

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 {