[Clang] prevent crash on invalid nested name specifiers with a single colon

clang/docs/ReleaseNotes.rst

@@ -737,6 +737,8 @@ 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
 ^^^^^^^^^^^^

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

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 '}'}}