[gcc r15-1256] libstdc++: Improve diagnostics for invalid std::hash specializations [PR115420]

[gcc r15-1256] libstdc++: Improve diagnostics for invalid std::hash specializations [PR115420]

[Jonathan Wakely]

https://gcc.gnu.org/g:cc38bdf093c44918edff819ae6c73d03c726b341

commit r15-1256-gcc38bdf093c44918edff819ae6c73d03c726b341
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jun 11 11:08:12 2024 +0100

    libstdc++: Improve diagnostics for invalid std::hash specializations [PR115420]
    
    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.

Diff:
---
 libstdc++-v3/include/bits/hashtable.h                    |  2 ++
 .../testsuite/23_containers/unordered_map/115420.cc      | 16 ++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/libstdc++-v3/include/bits/hashtable.h b/libstdc++-v3/include/bits/hashtable.h
index 983aa909d6c7..361da2b3b4d4 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 000000000000..5528bf813bd7
--- /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" }