[clang] Invalidate the alias template decl if it has multiple written

clang/lib/Sema/SemaDeclCXX.cpp

@@ -13588,6 +13588,7 @@ Decl *Sema::ActOnAliasDeclaration(Scope *S, AccessSpecifier AS,
       Diag(UsingLoc, diag::err_alias_template_extra_headers)
         << SourceRange(TemplateParamLists[1]->getTemplateLoc(),
          TemplateParamLists[TemplateParamLists.size()-1]->getRAngleLoc());
+      Invalid = true;
     }
     TemplateParameterList *TemplateParams = TemplateParamLists[0];
 

clang/lib/Sema/SemaTemplate.cpp

@@ -2728,6 +2728,8 @@ bool hasDeclaredDeductionGuides(DeclarationName Name, DeclContext *DC) {
 // Build deduction guides for a type alias template.
 void DeclareImplicitDeductionGuidesForTypeAlias(
     Sema &SemaRef, TypeAliasTemplateDecl *AliasTemplate, SourceLocation Loc) {
+  if (AliasTemplate->isInvalidDecl())
+    return;
   auto &Context = SemaRef.Context;
   // FIXME: if there is an explicit deduction guide after the first use of the
   // type alias usage, we will not cover this explicit deduction guide. fix this

clang/test/AST/ast-dump-invalid.cpp

@@ -60,3 +60,12 @@ double Str::foo1(double, invalid_type)
 // CHECK-NEXT:     `-ReturnStmt {{.*}} <col:3, col:10>
 // CHECK-NEXT:       `-ImplicitCastExpr {{.*}} <col:10> 'double' <IntegralToFloating>
 // CHECK-NEXT:         `-IntegerLiteral {{.*}} <col:10> 'int' 45
+
+namespace TestAliasTemplateDecl {
+template<typename T> class A;
+
+template<typename T>
+template<typename U> using InvalidAlias = A<U>;
+// CHECK:      TypeAliasTemplateDecl {{.*}} invalid InvalidAlias
+// CHECK-NEXT: |-TemplateTypeParmDecl {{.*}} typename depth 0 index 0 T
+}

clang/test/SemaCXX/cxx20-ctad-type-alias.cpp

@@ -247,3 +247,15 @@ using Bar = Foo<U>; // expected-note {{could not match 'Foo<type-parameter-0-0>'
 
 Bar s = {1}; // expected-error {{no viable constructor or deduction guide for deduction of template arguments}}
 } // namespace test18
+
+// GH85406, verify no crash on invalid alias templates.
+namespace test19 {
+template <typename T>
+class Foo {};
+
+template <typename T>
+template <typename K>
+using Bar2 = Foo<K>; // expected-error {{extraneous template parameter list in alias template declaration}}
+
+Bar2 b = 1; // expected-error {{no viable constructor or deduction guide for deduction of template arguments}}
+} // namespace test19