[clang][Parser] Fix crash on malformed using declaration in constexpr function

clang/docs/ReleaseNotes.rst

@@ -704,6 +704,7 @@ Bug Fixes in This Version
 - Fixed a bug with constexpr evaluation for structs containing unions in case of C++ modules. (#GH143168)
 - Fixed incorrect token location when emitting diagnostics for tokens expanded from macros. (#GH143216)
 - Fixed an infinite recursion when checking constexpr destructors. (#GH141789)
+- Fixed a crash when a malformed using declaration appears in a ``constexpr`` function. (#GH144264)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Parse/ParseDeclCXX.cpp

@@ -760,6 +760,10 @@ Parser::DeclGroupPtrTy Parser::ParseUsingDeclaration(
 
     Decl *AD = ParseAliasDeclarationAfterDeclarator(
         TemplateInfo, UsingLoc, D, DeclEnd, AS, Attrs, &DeclFromDeclSpec);
+
+    if (!AD)
+      return nullptr;
+
     return Actions.ConvertDeclToDeclGroup(AD, DeclFromDeclSpec);
   }