Backport/20.x: [Clang] Fix an incorrect assumption on getTemplatedDecl()

This backports d911085 because it fixes a regression introduced in 19
and we don't want it to persist in 20

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -1059,6 +1059,7 @@ Bug Fixes to C++ Support
   corresponding to a pack parameter (#GH124715)
 - Clang is now better at keeping track of friend function template instance contexts. (#GH55509)
 - Fixed an integer overflow bug in computing template parameter depths when synthesizing CTAD guides. (#GH128691)
+- Fixed an incorrect pointer access when checking access-control on concepts. (#GH131530)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaAccess.cpp

@@ -1518,8 +1518,8 @@ 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)) {
-    if (isa<DeclContext>(TD->getTemplatedDecl()))
-      DC = cast<DeclContext>(TD->getTemplatedDecl());
+    if (auto *D = dyn_cast_if_present<DeclContext>(TD->getTemplatedDecl()))
+      DC = D;
   } else if (auto *RD = dyn_cast<RequiresExprBodyDecl>(D)) {
     DC = RD;
   }

clang/test/SemaCXX/concept-crash-on-diagnostic.cpp

@@ -36,3 +36,15 @@ void function() {
 // expected-note@#4 {{candidate template ignored: constraints not satisfied [with IteratorL = Object *, IteratorR = Object *]}}
 // We don't know exactly the substituted type for `lhs == rhs`, thus a placeholder 'expr-type' is emitted.
 // expected-note@#3 {{because 'convertible_to<expr-type, bool>' would be invalid}}
+
+namespace GH131530 {
+
+class foo {
+  struct bar {}; // expected-note {{implicitly declared private}}
+};
+
+template <typename T>
+concept is_foo_concept = __is_same(foo::bar, T);
+// expected-error@-1 {{'bar' is a private member of 'GH131530::foo'}}
+
+}