Don't crash when code completing `using enum ^Foo`.

[sammccall]

Summary

Fixes https://github.com/clangd/clangd/issues/1281

[sammccall]

clang/lib/Parse/ParseDecl.cpp

4596

this is a bit unusual, usually parse() functions just stop after hitting CC, as if they hit something unexpected.

However ParseUsingDeclaration immediately calls ActOnUsingDeclaration, which asserts that the enum specifier we parsed was one of {enum, typename, error}. It seems simpler to signal failure using the existing mechanism than add a new one - LMK what you think.

[urnathan]

Might be mooted by fixing https://github.com/llvm/llvm-project/issues/57659, which I am working on

[sammccall]

In D134243#3802457, @urnathan wrote:

Might be mooted by fixing https://github.com/llvm/llvm-project/issues/57659, which I am working on

Great! Agree that larger changes around using-enum parsing would probably end up solving this in some better way.

Do you mind if I land it anyway? The testcase is useful even if the impl changes.
(Plus this is crashing for us in production, so just in case that larger change takes longer...)

[ilya-biryukov]

LGTM to fix the crash.
If @urnathan's changes happen to make this change obsolete, we could also revert consider reverting it afterwards.

clang/lib/Parse/ParseDecl.cpp

4596

Could you add a comment mentioning that ActOnUsingDeclaration will break if we don't do this?

clang/lib/Parse/ParseDecl.cpp

 void Parser::ParseEnumSpecifier(SourceLocation StartLoc, DeclSpec &DS,
                                 const ParsedTemplateInfo &TemplateInfo,
                                 AccessSpecifier AS, DeclSpecContext DSC) {
   // Parse the tag portion of this.
   if (Tok.is(tok::code_completion)) {
     // Code completion for an enum name.
     cutOffParsing();
     Actions.CodeCompleteTag(getCurScope(), DeclSpec::TST_enum);
+    DS.SetTypeSpecError(); // Needed by ActOnUsingDeclaration.
     return;
   }
 
   // If attributes exist after tag, parse them.
   ParsedAttributes attrs(AttrFactory);
   MaybeParseAttributes(PAKM_GNU | PAKM_Declspec | PAKM_CXX11, attrs);
 
   SourceLocation ScopedEnumKWLoc;

clang/test/CodeCompletion/using-enum.cpp

+enum class AAA { X, Y, Z };
+
+namespace N2 {
+  using enum AAA;
+  // RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:4:14 %s | FileCheck -check-prefix=CHECK-CC1 %s
+  // CHECK-CC1: COMPLETION: AAA
+};