[clang] Don't segfault on incorrect using directive (PR41400)

Summary:
this is a bugfixe for [[ https://bugs.llvm.org/show_bug.cgi?id=41400 | PR41400 ]]

added nullptr check at the relevent place and test

Reviewers: rsmith, riccibruno

Reviewed By: rsmith

Subscribers: jkooker, jkorous, riccibruno, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60523

llvm-svn: 363360

clang/lib/Sema/SemaExprCXX.cpp

@@ -90,7 +90,7 @@ ParsedType Sema::getConstructorName(IdentifierInfo &II,
   // When naming a constructor as a member of a dependent context (eg, in a
   // friend declaration or an inherited constructor declaration), form an
   // unresolved "typename" type.
-  if (CurClass->isDependentContext() && !EnteringContext) {
+  if (CurClass->isDependentContext() && !EnteringContext && SS.getScopeRep()) {
     QualType T = Context.getDependentNameType(ETK_None, SS.getScopeRep(), &II);
     return ParsedType::make(T);
   }

clang/test/SemaCXX/using-decl-1.cpp

@@ -396,3 +396,10 @@ namespace tag_vs_var {
   using N::Y;
   using N::Z;
 }
+
+// expected-error@+5 {{requires a qualified name}}
+// expected-error@+4 {{expected ';'}}
+// expected-error@+3 {{expected '}'}}
+// expected-note@+2 {{to match this '{'}}
+// expected-error@+1 {{expected ';'}}
+template<class> struct S { using S