[clang] [Clang][Sema] Differentiate between partial/explicit specializations when diagnosing unexpanded packs (PR #72015)

[Erich Keane via cfe-commits]

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@@ -369,9 +369,39 @@ void test_unexpanded_exprs(Types ...values) {
   void f(int arg = values); // expected-error{{default argument contains unexpanded parameter pack 'values'}}
 }
 
-// Test unexpanded parameter packs in partial specializations.
-template<typename ...Types>
-struct TestUnexpandedDecls<int, Types>; // expected-error{{partial specialization contains unexpanded parameter pack 'Types'}}
+// Test unexpanded parameter packs in partial/explicit specializations.
+namespace Specializations {
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erichkeane wrote:

This test is not showing any change in behavior? All of these diagnostics are existing behavior (https://godbolt.org/z/xr49ac7Ed). What actual changes does this patch make?

https://github.com/llvm/llvm-project/pull/72015