Improve diagnostics when an invalid hash is used in an unordered container.

This patch adds a static assertion that the specified hash meets
the requirements of an enabled hash, and it ensures that the static
assertion is evaluated before __compressed_pair is instantiated.
That way the static assertion diagnostic is emitted first.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@296565 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/containers/unord/unord.set/missing_hash_specialization.fail.cpp

@@ -0,0 +1,32 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+// REQUIRES: diagnose-if-support
+
+// <unordered_set>
+
+// Test that we generate a reasonable diagnostic when the specified hash is
+// not enabled.
+
+#include <unordered_set>
+#include <utility>
+
+using VT = std::pair<int, int>;
+using Set = std::unordered_set<VT>;
+
+int main() {
+
+  Set s; // expected-error@__hash_table:* {{the specified hash functor does not meet the requirements for an enabled hash}}
+
+  // FIXME: It would be great to suppress the below diagnostic all together.
+  //        but for now it's sufficient that it appears last. However there is
+  //        currently no way to test the order diagnostics are issued.
+  // expected-error@memory:* {{call to implicitly-deleted default constructor of 'std::__1::hash<std::__1::pair<int, int> >'}}
+}