[clang] Fix crash upon stray coloncolon token in C2x mode

The parser assumes that the lexer never emits coloncolon token for C code, but this assumption no longer holds in C2x attribute namespaces. As a result, stray coloncolon tokens out of attributes cause assertion failures and hangs in release build, which this patch tries to handle.

Crash input minimal example: `T n::v`

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D133248

clang/docs/ReleaseNotes.rst

@@ -251,6 +251,7 @@ Bug Fixes
 - Address the thread identification problems in coroutines.
   `Issue 47177 <https://github.com/llvm/llvm-project/issues/47177>`_
   `Issue 47179 <https://github.com/llvm/llvm-project/issues/47179>`_
+- Fix a crash upon stray coloncolon token in C2x mode.
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Parse/Parser.h

@@ -866,10 +866,11 @@ public:
   bool TryAnnotateCXXScopeToken(bool EnteringContext = false);
 
   bool MightBeCXXScopeToken() {
-    return Tok.is(tok::identifier) || Tok.is(tok::coloncolon) ||
-           (Tok.is(tok::annot_template_id) &&
-            NextToken().is(tok::coloncolon)) ||
-           Tok.is(tok::kw_decltype) || Tok.is(tok::kw___super);
+    return getLangOpts().CPlusPlus &&
+           (Tok.is(tok::identifier) || Tok.is(tok::coloncolon) ||
+            (Tok.is(tok::annot_template_id) &&
+             NextToken().is(tok::coloncolon)) ||
+            Tok.is(tok::kw_decltype) || Tok.is(tok::kw___super));
   }
   bool TryAnnotateOptionalCXXScopeToken(bool EnteringContext = false) {
     return MightBeCXXScopeToken() && TryAnnotateCXXScopeToken(EnteringContext);

clang/lib/Parse/ParseDecl.cpp

@@ -5411,6 +5411,8 @@ bool Parser::isDeclarationSpecifier(
     return isDeclarationSpecifier(AllowImplicitTypename);
 
   case tok::coloncolon:   // ::foo::bar
+    if (!getLangOpts().CPlusPlus)
+      return false;
     if (NextToken().is(tok::kw_new) ||    // ::new
         NextToken().is(tok::kw_delete))   // ::delete
       return false;

clang/lib/Parse/Parser.cpp

@@ -2080,9 +2080,9 @@ bool Parser::TryAnnotateTypeOrScopeTokenAfterScopeSpec(
     }
 
     if (!getLangOpts().CPlusPlus) {
-      // If we're in C, we can't have :: tokens at all (the lexer won't return
-      // them).  If the identifier is not a type, then it can't be scope either,
-      // just early exit.
+      // If we're in C, the only place we can have :: tokens is C2x
+      // attribute which is parsed elsewhere. If the identifier is not a type,
+      // then it can't be scope either, just early exit.
       return false;
     }
 

clang/test/Parser/c2x-attributes.c

@@ -141,3 +141,6 @@ void test_asm(void) {
 struct [[]] S4 *s; // expected-error {{an attribute list cannot appear here}}
 struct S5 {};
 int c = sizeof(struct [[]] S5); // expected-error {{an attribute list cannot appear here}}
+
+// Ensure that '::' outside of attributes does not crash and is not treated as scope
+double n::v; // expected-error {{expected ';' after top level declarator}}