diff --git a/clang/include/clang/Parse/Parser.h b/clang/include/clang/Parse/Parser.h
index c4bef4729fd36..90f17465bce7d 100644
--- a/clang/include/clang/Parse/Parser.h
+++ b/clang/include/clang/Parse/Parser.h
@@ -8909,6 +8909,8 @@ class Parser : public CodeCompletionHandler {
                             bool OuterMightBeMessageSend = false);
 
   ///@}
+
+  TemplateParameterLists *TemplateParamsFromAlias = nullptr;
 };
 
 } // end namespace clang
diff --git a/clang/lib/Parse/ParseDeclCXX.cpp b/clang/lib/Parse/ParseDeclCXX.cpp
index 2cf33a856c4f4..4069d257b0150 100644
--- a/clang/lib/Parse/ParseDeclCXX.cpp
+++ b/clang/lib/Parse/ParseDeclCXX.cpp
@@ -886,11 +886,17 @@ Decl *Parser::ParseAliasDeclarationAfterDeclarator(
         << FixItHint::CreateRemoval(SourceRange(D.EllipsisLoc));
 
   Decl *DeclFromDeclSpec = nullptr;
+
+  this->TemplateParamsFromAlias = TemplateInfo.TemplateParams;
+
   TypeResult TypeAlias =
       ParseTypeName(nullptr,
                     TemplateInfo.Kind != ParsedTemplateKind::NonTemplate ? DeclaratorContext::AliasTemplate
                                       : DeclaratorContext::AliasDecl,
                     AS, &DeclFromDeclSpec, &Attrs);
+
+  this->TemplateParamsFromAlias = nullptr;
+
   if (OwnedType)
     *OwnedType = DeclFromDeclSpec;
 
@@ -2173,6 +2179,17 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
     assert(Tok.is(tok::l_brace) ||
            (getLangOpts().CPlusPlus && Tok.is(tok::colon)) ||
            isClassCompatibleKeyword());
+
+    if (TemplateParamsFromAlias) {
+      for (const TemplateParameterList *TPL : *TemplateParamsFromAlias) {
+        for (NamedDecl *D : *TPL) {
+          D->setInvalidDecl(true);
+          auto *TTPD = dyn_cast<TemplateTypeParmDecl>(D);
+          TTPD->setTypeForDecl(Actions.Context.IntTy.getTypePtr());
+        }
+      }
+    }
+
     if (SkipBody.ShouldSkip)
       SkipCXXMemberSpecification(StartLoc, AttrFixitLoc, TagType,
                                  TagOrTempResult.get());
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 86b871396ec90..50c09bd3cdea7 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -288,8 +288,8 @@ static ParsedType buildNamedType(Sema &S, const CXXScopeSpec *SS, QualType T,
   case Type::ObjCTypeParam:
   case Type::TemplateTypeParm:
     return ParsedType::make(T);
-  default:
-    llvm_unreachable("Unexpected Type Class");
+  //default:
+    //llvm_unreachable("Unexpected Type Class");
   }
 
   if (!SS || SS->isEmpty())
diff --git a/clang/test/SemaCXX/concept-crash-on-diagnostic.cpp b/clang/test/SemaCXX/concept-crash-on-diagnostic.cpp
index 1efed72522fef..fa3eb1372bd3f 100644
--- a/clang/test/SemaCXX/concept-crash-on-diagnostic.cpp
+++ b/clang/test/SemaCXX/concept-crash-on-diagnostic.cpp
@@ -60,3 +60,16 @@ concept atomicish = requires() {
 };
 atomicish<int> f(); // expected-error {{expected 'auto' or 'decltype(auto)' after concept name}}
 } // namespace GH138820
+
+namespace GH91564 {
+template <class T> using A = struct B { // expected-error {{'GH91564::B' cannot be defined in a type alias template}}
+  template <class> void f() requires (T()); // expected-error {{atomic constraint must be of type 'bool' (found 'int')}} expected-note {{explicit instantiation refers here}}
+};
+template void B::f<void>(); // expected-error {{explicit instantiation of undefined function template 'f'}}
+
+template <class T> using C = struct D { // expected-error {{'GH91564::D' cannot be defined in a type alias template}}
+  using E = T;
+};
+template <class> void g() requires (D::E()); // expected-error {{atomic constraint must be of type 'bool' (found 'D::E' (aka 'int'))}} expected-note {{explicit instantiation refers here}}
+template void g<void>(); // expected-error {{explicit instantiation of undefined function template 'g'}}
+}
\ No newline at end of file