Add a regression test for recent regression in pair. (#97355)

PR #96165 broke code similar to this test, and was subsequently
reverted. Add a test-case, to ensure the problem won't reoccur.

This error is potentially related to issues #59292 and #59966.

libcxx/test/libcxx/utilities/utility/pairs/pairs.pair/pair.incomplete.compile.pass.cpp

@@ -0,0 +1,22 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Check that instantiating pair doesn't look up type traits "too early", before
+// the contained types have been completed.
+//
+// This is a regression test, to prevent a reoccurrance of the issue introduced
+// in 5e1de27f680591a870d78e9952b23f76aed7f456.
+
+#include <utility>
+#include <vector>
+
+struct Test {
+  std::vector<std::pair<int, Test> > v;
+};
+
+std::pair<int, Test> p;