[clang] Fix an assertion crash in delayed access check.

`TD->getTemplatedDecl()` might not be a DeclContext variant, which can
trigger an assertion inside `isa<>`.

Differential Revision: https://reviews.llvm.org/D91380

clang/lib/Sema/SemaAccess.cpp

@@ -1476,7 +1476,8 @@ void Sema::HandleDelayedAccessCheck(DelayedDiagnostic &DD, Decl *D) {
   } else if (FunctionDecl *FN = dyn_cast<FunctionDecl>(D)) {
     DC = FN;
   } else if (TemplateDecl *TD = dyn_cast<TemplateDecl>(D)) {
-    DC = cast<DeclContext>(TD->getTemplatedDecl());
+    if (isa<DeclContext>(TD->getTemplatedDecl()))
+      DC = cast<DeclContext>(TD->getTemplatedDecl());
   }
 
   EffectiveContext EC(DC);

clang/test/SemaCXX/cxx14-access.cpp

@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++14 -verify %s
+
+namespace NoCrashOnDelayedAccessCheck {
+class Foo {
+  class Private; // expected-note {{declared private here}}
+};
+
+struct Bar {};
+
+template <typename T>
+Foo::Private Bar::ABC; // expected-error {{no member named 'ABC' in 'NoCrashOnDelayedAccessCheck::Bar'}} \
+                          expected-error {{'Private' is a private member of}}
+}