[clang] [clang] Fix infinite loop when parsing `::` in C mode (#208044) (PR #209554)

[Macro Terra via cfe-commits]

https://github.com/hongtaihu updated https://github.com/llvm/llvm-project/pull/209554

>From f12ccf82cb4193900a58639ac5e796c447095aaf Mon Sep 17 00:00:00 2001
From: hongtaihu <13541229370 at qq.com>
Date: Wed, 15 Jul 2026 00:48:22 +0800
Subject: [PATCH] [clang] Fix infinite loop when parsing `::` in C mode
 (#208044)

In `isTypeSpecifierQualifier`, the `case tok::coloncolon` tries to
annotate `::` as a C++ scope token via `TryAnnotateTypeOrScopeToken`.
In C mode, `ParseOptionalCXXScopeSpecifier` is gated on `CPlusPlus`,
so the annotation does nothing and the token remains `::`.  The code
then recursively calls `isTypeSpecifierQualifier(getCurToken())`,
hitting the same `::` token again in an infinite loop.

Fix by returning `false` when the current token is still `::` after
the failed annotation attempt, cleanly answering "this is not a type
specifier qualifier" and breaking the cycle.

Fixes https://github.com/llvm/llvm-project/issues/208044
---
 clang/lib/Parse/ParseDecl.cpp         | 2 ++
 clang/test/Parser/coloncolon-c-mode.c | 4 ++++
 2 files changed, 6 insertions(+)
 create mode 100644 clang/test/Parser/coloncolon-c-mode.c

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 88f07bb104fcb..335dd4087b687 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -5692,6 +5692,8 @@ bool Parser::isTypeSpecifierQualifier(const Token &Tok) {
 
     if (TryAnnotateTypeOrScopeToken())
       return true;
+    if (getCurToken().is(tok::coloncolon))
+      return false;
     return isTypeSpecifierQualifier(getCurToken());
 
     // GNU attributes support.
diff --git a/clang/test/Parser/coloncolon-c-mode.c b/clang/test/Parser/coloncolon-c-mode.c
new file mode 100644
index 0000000000000..832655825530a
--- /dev/null
+++ b/clang/test/Parser/coloncolon-c-mode.c
@@ -0,0 +1,4 @@
+// Regression test for GH-208044: clang should not hang when parsing
+// `::` in C mode.
+// RUN: %clang_cc1 -x c -std=c23 -fsyntax-only %s
+int sink = (::i);