[gcc r12-6827] libstdc++: Restore support for unordered_map<const T, ...> [PR104174]

[gcc r12-6827] libstdc++: Restore support for unordered_map<const T, ...> [PR104174]

[Jonathan Wakely]

https://gcc.gnu.org/g:084680db9af077ca37c5523a58b6c11e090e7335

commit r12-6827-g084680db9af077ca37c5523a58b6c11e090e7335
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Sat Jan 22 00:35:45 2022 +0000

    libstdc++: Restore support for unordered_map<const T, ...> [PR104174]
    
    I broke this unintentionally in r12-4259.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/104174
            * include/bits/hashtable_policy.h (_Map_base): Add partial
            specialization for maps with const key types.
            * testsuite/23_containers/unordered_map/104174.cc: New test.

Diff:
---
 libstdc++-v3/include/bits/hashtable_policy.h                 | 11 +++++++++++
 libstdc++-v3/testsuite/23_containers/unordered_map/104174.cc |  4 ++++
 2 files changed, 15 insertions(+)

diff --git a/libstdc++-v3/include/bits/hashtable_policy.h b/libstdc++-v3/include/bits/hashtable_policy.h
index 3b60eb9ae72..0f0b0f9ea51 100644
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -812,6 +812,17 @@ namespace __detail
       return __pos->second;
     }
 
+  // Partial specialization for unordered_map<const T, U>, see PR 104174.
+  template<typename _Key, typename _Val, typename _Alloc, typename _Equal,
+	   typename _Hash, typename _RangeHash, typename _Unused,
+	   typename _RehashPolicy, typename _Traits, bool __uniq>
+    struct _Map_base<const _Key, pair<const _Key, _Val>,
+		     _Alloc, _Select1st, _Equal, _Hash,
+		     _RangeHash, _Unused, _RehashPolicy, _Traits, __uniq>
+    : _Map_base<_Key, pair<const _Key, _Val>, _Alloc, _Select1st, _Equal, _Hash,
+		_RangeHash, _Unused, _RehashPolicy, _Traits, __uniq>
+    { };
+
   /**
    *  Primary class template _Insert_base.
    *
diff --git a/libstdc++-v3/testsuite/23_containers/unordered_map/104174.cc b/libstdc++-v3/testsuite/23_containers/unordered_map/104174.cc
new file mode 100644
index 00000000000..4007425bf74
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/unordered_map/104174.cc
@@ -0,0 +1,4 @@
+// { dg-do compile { target c++11 } }
+// PR libstdc++/104174 unordered_map<const T, U, H> fails
+#include <unordered_map>
+std::unordered_map<const int, int, std::hash<int>> m;