[clang] Fix crash on visiting null nestedNameSpecifier.

Summary: Fix https://github.com/clangd/clangd/issues/293

Reviewers: sammccall

Subscribers: ilya-biryukov, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76320

(cherry picked from commit bd763e2cf7c1d84bab95064cc5cbe542b227b025)

2 files changed, 10 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 943e8f422a7..264c903209a 100755
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -5928,7 +5928,9 @@ bool UnnamedLocalNoLinkageFinder::VisitDependentNameType(
 
 bool UnnamedLocalNoLinkageFinder::VisitDependentTemplateSpecializationType(
                                  const DependentTemplateSpecializationType* T) {
-  return VisitNestedNameSpecifier(T->getQualifier());
+  if (auto *Q = T->getQualifier())
+    return VisitNestedNameSpecifier(Q);
+  return false;
 }
 
 bool UnnamedLocalNoLinkageFinder::VisitPackExpansionType(
@@ -5982,6 +5984,7 @@ bool UnnamedLocalNoLinkageFinder::VisitTagDecl(const TagDecl *Tag) {
 
 bool UnnamedLocalNoLinkageFinder::VisitNestedNameSpecifier(
                                                     NestedNameSpecifier *NNS) {
+  assert(NNS);
   if (NNS->getPrefix() && VisitNestedNameSpecifier(NNS->getPrefix()))
     return true;
 
diff --git a/clang/test/Parser/cxx-template-decl.cpp b/clang/test/Parser/cxx-template-decl.cpp
index 3d7a3dc14f4..0d52ad8fb50 100644
--- a/clang/test/Parser/cxx-template-decl.cpp
+++ b/clang/test/Parser/cxx-template-decl.cpp
@@ -273,3 +273,9 @@ namespace AnnotateAfterInvalidTemplateId {
 namespace PR45063 {
   template<class=class a::template b<>> struct X {}; // expected-error {{undeclared identifier 'a'}}
 }
+
+namespace NoCrashOnEmptyNestedNameSpecifier {
+  template <typename FnT,
+            typename T = typename ABC<FnT>::template arg_t<0>> // expected-error {{no template named 'ABC'}}
+  void foo(FnT) {}
+}