Worked around GCC bug 56480. Explicit specialization in a different namespace.

[mpark]

Summary

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56480

libcxx/trunk/test/std/utilities/optional/optional.hash/hash.pass.cpp

 #include <memory>
 #include <cassert>
 
 #include "poisoned_hash_helper.hpp"
 
 struct A {};
 struct B {};
 
+namespace std {
+
 template <>
-struct std::hash<B> {
+struct hash<B> {
   size_t operator()(B const&) { return 0; }
 };
 
+}
+
 int main()
 {
     using std::optional;
     const std::size_t nullopt_hash =
         std::hash<optional<double>>{}(optional<double>{});
 
     {
         typedef int T;

libcxx/trunk/test/std/utilities/variant/variant.hash/hash.pass.cpp

void test_hash_variant_duplicate_elements() {
     assert(h(v1) == h(v1));
     assert(h(v2) == h(v2));
     LIBCPP_ASSERT(h(v1) != h(v2));
 }
 
 struct A {};
 struct B {};
 
+namespace std {
+
 template <>
-struct std::hash<B> {
+struct hash<B> {
   size_t operator()(B const&) const {
     return 0;
   }
 };
 
+}
+
 void test_hash_variant_enabled() {
   {
     test_hash_enabled_for_type<std::variant<int> >();
     test_hash_enabled_for_type<std::variant<int*, long, double, const int> >();
   }
   {
     test_hash_disabled_for_type<std::variant<int, A>>();
     test_hash_disabled_for_type<std::variant<const A, void*>>();