add tests to verify handling of invalid alias resolution

Author: a-tarasyuk

clang/lib/Sema/SemaDeclCXX.cpp

@@ -11454,25 +11454,26 @@ bool Sema::CheckDeductionGuideDeclarator(Declarator &D, QualType &R,
       const TemplateSpecializationType *TST = RetTST.getTypePtr();
       while (TST && TST->isTypeAlias())
         TST = TST->getAliasedType()->getAs<TemplateSpecializationType>();
-      assert(TST && "failed to resolve type alias");
-
-      TemplateName SpecifiedName = TST->getTemplateName();
-      bool TemplateMatches = Context.hasSameTemplateName(
-          SpecifiedName, GuidedTemplate, /*IgnoreDeduced=*/true);
-
-      const QualifiedTemplateName *Qualifiers =
-          SpecifiedName.getAsQualifiedTemplateName();
-      assert(Qualifiers && "expected QualifiedTemplate");
-      bool SimplyWritten = !Qualifiers->hasTemplateKeyword() &&
-                           Qualifiers->getQualifier() == nullptr;
-      if (SimplyWritten && TemplateMatches)
-        AcceptableReturnType = true;
-      else {
-        // This could still instantiate to the right type, unless we know it
-        // names the wrong class template.
-        auto *TD = SpecifiedName.getAsTemplateDecl();
-        MightInstantiateToSpecialization = !(TD && isa<ClassTemplateDecl>(TD) &&
-                                             !TemplateMatches);
+
+      if (TST) {
+        TemplateName SpecifiedName = TST->getTemplateName();
+        bool TemplateMatches = Context.hasSameTemplateName(
+            SpecifiedName, GuidedTemplate, /*IgnoreDeduced=*/true);
+
+        const QualifiedTemplateName *Qualifiers =
+            SpecifiedName.getAsQualifiedTemplateName();
+        assert(Qualifiers && "expected QualifiedTemplate");
+        bool SimplyWritten = !Qualifiers->hasTemplateKeyword() &&
+                             Qualifiers->getQualifier() == nullptr;
+        if (SimplyWritten && TemplateMatches)
+          AcceptableReturnType = true;
+        else {
+          // This could still instantiate to the right type, unless we know it
+          // names the wrong class template.
+          auto *TD = SpecifiedName.getAsTemplateDecl();
+          MightInstantiateToSpecialization =
+              !(TD && isa<ClassTemplateDecl>(TD) && !TemplateMatches);
+        }
       }
     } else if (!RetTy.hasQualifiers() && RetTy->isDependentType()) {
       MightInstantiateToSpecialization = true;

clang/test/CXX/temp/temp.deduct.guide/p3.cpp

@@ -71,3 +71,11 @@ namespace WrongScope {
     Local(int) -> Local<int>; // expected-error {{expected}}
   }
 }
+
+namespace GH54909 {
+template <class T> struct A {};
+struct B {};
+
+template <typename T> using C = B;
+template <typename T> A() -> C<T>; // expected-error {{deduced type 'C<T>' (aka 'GH54909::B') of deduction guide is not a specialization of template 'A'}}
+}