[clang] [clang] Fix possible nullptr deref in BuildCXXNestedNameSpecifier (PR #166995)

[Paul Kirth via cfe-commits]

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@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -fsyntax-only %s -verify
+namespace a {
+template <class b>
+void c() {
+  ((::c::)); // expected-error {{expected unqualified-id}}
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ilovepi wrote:

i've updated the early exit by duplicating the diagnostic. I didn't see an obvious way to avoid doing that. Happy to amend if this still isn't quite right.

https://github.com/llvm/llvm-project/pull/166995