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

The issue occurred because the template parameter scope was skipped
too early, before diagnosing the alias name shadowing.

To fix this, the patch moves it to after LookupName, such that the behavior
remains consistent with the typedef implementation.

Fixes #123423

Author: BaLiKfromUA

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

@@ -54,18 +54,24 @@ namespace LookupFilter {
   template<typename U> using S = S<U>*; // ok
 }
 
-namespace InFunctions {
+namespace UnexpandedPack {
   template<typename...T> struct S0 {
     template<typename Z> using U = T*; // expected-error {{declaration type contains unexpanded parameter pack 'T'}}
     U<char> u;
   };
+}
 
+namespace InvalidType {
   template<typename Z> using T1 = int;
   template<typename Z> using T2 = int[-1]; // expected-error {{array size is negative}}
+}
+
+namespace ShadowTemplateParam {
   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 {