https://github.com/a-tarasyuk updated 
https://github.com/llvm/llvm-project/pull/169246

>From 0645128caa40e835866720d7ea481be21e007cff Mon Sep 17 00:00:00 2001
From: Oleksandr Tarasiuk <[email protected]>
Date: Sun, 23 Nov 2025 22:56:50 +0200
Subject: [PATCH] [Clang] prevent crash on invalid nested name specifiers with
 a single colon

---
 clang/docs/ReleaseNotes.rst                |  2 ++
 clang/lib/Parse/ParseTentative.cpp         |  3 ++-
 clang/test/Parser/cxx-nested-name-spec.cpp | 10 ++++++++++
 3 files changed, 14 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/Parser/cxx-nested-name-spec.cpp

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 63930f43c25e3..5b481dc9ae249 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -719,6 +719,8 @@ Crash and bug fixes
   ``[[assume(expr)]]`` attribute was enclosed in parentheses.  (#GH151529)
 - Fixed a crash when parsing ``#embed`` parameters with unmatched closing 
brackets. (#GH152829)
 - Fixed a crash when compiling ``__real__`` or ``__imag__`` unary operator on 
scalar value with type promotion. (#GH160583)
+- Fixed a crash when parsing invalid nested name specifier sequences
+  containing a single colon. (#GH167905)
 
 Improvements
 ^^^^^^^^^^^^
diff --git a/clang/lib/Parse/ParseTentative.cpp 
b/clang/lib/Parse/ParseTentative.cpp
index 82f2294ff5bb7..75a582e70b244 100644
--- a/clang/lib/Parse/ParseTentative.cpp
+++ b/clang/lib/Parse/ParseTentative.cpp
@@ -1063,7 +1063,8 @@ Parser::isCXXDeclarationSpecifier(ImplicitTypenameContext 
AllowImplicitTypename,
       return TPResult::False;
     }
 
-    if (Next.isNot(tok::coloncolon) && Next.isNot(tok::less)) {
+    if (Next.isNot(tok::coloncolon) && Next.isNot(tok::less) &&
+        Next.isNot(tok::colon)) {
       // Determine whether this is a valid expression. If not, we will hit
       // a parse error one way or another. In that case, tell the caller that
       // this is ambiguous. Typo-correct to type and expression keywords and
diff --git a/clang/test/Parser/cxx-nested-name-spec.cpp 
b/clang/test/Parser/cxx-nested-name-spec.cpp
new file mode 100644
index 0000000000000..3a551a4f2221f
--- /dev/null
+++ b/clang/test/Parser/cxx-nested-name-spec.cpp
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+namespace a { b c ( a:c::
+// expected-error@-1 {{unknown type name 'b'}}
+// expected-error@-2 {{unexpected ':' in nested name specifier; did you mean 
'::'?}}
+// expected-error@-3 {{no member named 'c' in namespace 'a'}}
+// expected-error@-4 {{expected ';' after top level declarator}}
+// expected-note@-5 {{to match this '{'}}
+// expected-error@+1 {{expected unqualified-id}} \
+// expected-error@+1 {{expected '}'}}

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