This should be safe to test when the class is instantiated (unlike
testing that the hash function is invocable, which we delay until it's
needed). A disabled std::hash specialization is not copy constructible,
and std::hash<std::string> for a forward-declared std::string is
disabled, so this greatly improves the error in the PR. I think it
should be an improvement more generally too.
Tested x86_64-linux.
-- >8 --
When using a key type without a valid std::hash specialization the
unordered containers give confusing diagnostics about the default
constructor being deleted. Add a static_assert that will fail for
disabled std::hash specializations (and for a subset of custom hash
functions).
libstdc++-v3/ChangeLog:
PR libstdc++/115420
* include/bits/hashtable.h (_Hashtable): Add static_assert to
check that hash function is copy constructible.
* testsuite/23_containers/unordered_map/115420.cc: New test.
---
libstdc++-v3/include/bits/hashtable.h | 2 ++
.../23_containers/unordered_map/115420.cc | 16 ++++++++++++++++
2 files changed, 18 insertions(+)
create mode 100644 libstdc++-v3/testsuite/23_containers/unordered_map/115420.cc
diff --git a/libstdc++-v3/include/bits/hashtable.h
b/libstdc++-v3/include/bits/hashtable.h
index 983aa909d6c..361da2b3b4d 100644
--- a/libstdc++-v3/include/bits/hashtable.h
+++ b/libstdc++-v3/include/bits/hashtable.h
@@ -210,6 +210,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static_assert(is_same<typename _Alloc::value_type, _Value>{},
"unordered container must have the same value_type as its allocator");
#endif
+ static_assert(is_copy_constructible<_Hash>::value,
+ "hash function must be copy constructible");
using __traits_type = _Traits;
using __hash_cached = typename __traits_type::__hash_cached;
diff --git a/libstdc++-v3/testsuite/23_containers/unordered_map/115420.cc
b/libstdc++-v3/testsuite/23_containers/unordered_map/115420.cc
new file mode 100644
index 00000000000..5528bf813bd
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/unordered_map/115420.cc
@@ -0,0 +1,16 @@
+// { dg-do compile { target c++11 } }
+
+#include <unordered_map>
+
+struct S { };
+
+void
+test_pr115420()
+{
+ std::unordered_map<S, int> m; // { dg-error "here" }
+}
+
+// { dg-error "hash function must be copy constructible" "" { target *-*-* } 0
}
+// { dg-prune-output "use of deleted function" }
+// { dg-prune-output "is private" }
+// { dg-prune-output "no matching function" }
--
2.45.1
