[clang] Fix a null-NSS-access crash in DependentNameType.

The DependentNameType must have a non-null NSS. This property could be
violated during typo correction.

Differential Revision: https://reviews.llvm.org/D82738

clang/lib/Sema/SemaTemplate.cpp

@@ -4844,10 +4844,7 @@ bool Sema::CheckTemplateTypeArgument(TemplateTypeParmDecl *Param,
     CXXScopeSpec SS;
     DeclarationNameInfo NameInfo;
 
-    if (DeclRefExpr *ArgExpr = dyn_cast<DeclRefExpr>(Arg.getAsExpr())) {
-      SS.Adopt(ArgExpr->getQualifierLoc());
-      NameInfo = ArgExpr->getNameInfo();
-    } else if (DependentScopeDeclRefExpr *ArgExpr =
+   if (DependentScopeDeclRefExpr *ArgExpr =
                dyn_cast<DependentScopeDeclRefExpr>(Arg.getAsExpr())) {
       SS.Adopt(ArgExpr->getQualifierLoc());
       NameInfo = ArgExpr->getNameInfo();
@@ -4866,6 +4863,7 @@ bool Sema::CheckTemplateTypeArgument(TemplateTypeParmDecl *Param,
       if (Result.getAsSingle<TypeDecl>() ||
           Result.getResultKind() ==
               LookupResult::NotFoundInCurrentInstantiation) {
+        assert(SS.getScopeRep() && "dependent scope expr must has a scope!");
         // Suggest that the user add 'typename' before the NNS.
         SourceLocation Loc = AL.getSourceRange().getBegin();
         Diag(Loc, getLangOpts().MSVCCompat

clang/test/Parser/cxx-template-decl.cpp

@@ -286,3 +286,17 @@ namespace PR45239 {
   template<int> int b;
   template<int> auto f() -> b<0>; // expected-error +{{}}
 }
+
+namespace NoCrashOnNullNNSTypoCorrection {
+
+int AddObservation(); // expected-note {{declared here}}
+
+template <typename T, typename... Args> // expected-note {{template parameter is declared here}}
+class UsingImpl {};
+class AddObservation {
+  using Using =
+    UsingImpl<AddObservationFn, const int>; // expected-error {{use of undeclared identifier 'AddObservationFn'; did you mean}} \
+                                               expected-error {{template argument for template type parameter must be a type}}
+};
+
+}