From 18411e5a853a34870a6d30d4bdabb84301ad99c1 Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@nuwen.net>
Date: Tue, 18 Apr 2023 12:30:43 -0700
Subject: [PATCH] Allow `MapLike<K, V&>` again, and test it.

---
 stl/inc/map                                      |  4 ----
 stl/inc/unordered_map                            |  4 ----
 .../test.compile.pass.cpp                        | 16 ++++++++++++++++
 3 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/stl/inc/map b/stl/inc/map
index eaca60e293..b7e1cb601c 100644
--- a/stl/inc/map
+++ b/stl/inc/map
@@ -77,8 +77,6 @@ public:
         _MISMATCHED_ALLOCATOR_MESSAGE("map<Key, Value, Compare, Allocator>", "pair<const Key, Value>"));
     static_assert(is_object_v<_Kty>, "The C++ Standard forbids containers of non-object types "
                                      "because of [container.requirements].");
-    static_assert(is_object_v<_Ty>, "The C++ Standard forbids containers of non-object types "
-                                    "because of [container.requirements].");
 
     using _Mybase                = _Tree<_Tmap_traits<_Kty, _Ty, _Pr, _Alloc, false>>;
     using _Nodeptr               = typename _Mybase::_Nodeptr;
@@ -463,8 +461,6 @@ public:
         _MISMATCHED_ALLOCATOR_MESSAGE("multimap<Key, Value, Compare, Allocator>", "pair<const Key, Value>"));
     static_assert(is_object_v<_Kty>, "The C++ Standard forbids containers of non-object types "
                                      "because of [container.requirements].");
-    static_assert(is_object_v<_Ty>, "The C++ Standard forbids containers of non-object types "
-                                    "because of [container.requirements].");
 
     using _Mybase                = _Tree<_Tmap_traits<_Kty, _Ty, _Pr, _Alloc, true>>;
     using key_type               = _Kty;
diff --git a/stl/inc/unordered_map b/stl/inc/unordered_map
index 40561f6eb7..dee66749bf 100644
--- a/stl/inc/unordered_map
+++ b/stl/inc/unordered_map
@@ -71,8 +71,6 @@ public:
         _MISMATCHED_ALLOCATOR_MESSAGE("unordered_map<Key, Value, Hasher, Eq, Allocator>", "pair<const Key, Value>"));
     static_assert(is_object_v<_Kty>, "The C++ Standard forbids containers of non-object types "
                                      "because of [container.requirements].");
-    static_assert(is_object_v<_Ty>, "The C++ Standard forbids containers of non-object types "
-                                    "because of [container.requirements].");
 
 private:
     using _Mytraits      = _Uhash_compare<_Kty, _Hasher, _Keyeq>;
@@ -563,8 +561,6 @@ public:
             "unordered_multimap<Key, Value, Hasher, Eq, Allocator>", "pair<const Key, Value>"));
     static_assert(is_object_v<_Kty>, "The C++ Standard forbids containers of non-object types "
                                      "because of [container.requirements].");
-    static_assert(is_object_v<_Ty>, "The C++ Standard forbids containers of non-object types "
-                                    "because of [container.requirements].");
 
 private:
     using _Mytraits      = _Uhash_compare<_Kty, _Hasher, _Keyeq>;
diff --git a/tests/std/tests/Dev11_0437519_container_requirements/test.compile.pass.cpp b/tests/std/tests/Dev11_0437519_container_requirements/test.compile.pass.cpp
index 466a658ba6..296ca936c4 100644
--- a/tests/std/tests/Dev11_0437519_container_requirements/test.compile.pass.cpp
+++ b/tests/std/tests/Dev11_0437519_container_requirements/test.compile.pass.cpp
@@ -3048,6 +3048,17 @@ void assert_container() {
     check_all_specific_requirements<Tag>();
 }
 
+// MapLike<K, V&> is squirrelly, but appears to be permitted.
+template <template <class...> class MapLike>
+void check_reference_as_mapped_type() {
+    double dbl = 3.14;
+
+    MapLike<int, double&> ml;
+    ml.emplace(10, dbl);
+
+    STATIC_ASSERT(std::is_same_v<decltype(ml.find(10)->second), double&>);
+}
+
 void assert_all() {
     assert_container<tag_deque>();
     assert_container<tag_forward_list>();
@@ -3063,4 +3074,9 @@ void assert_all() {
     assert_container<tag_unordered_multimap>();
     assert_container<tag_unordered_multiset>();
     assert_container<tag_unordered_set>();
+
+    check_reference_as_mapped_type<std::map>();
+    check_reference_as_mapped_type<std::multimap>();
+    check_reference_as_mapped_type<std::unordered_map>();
+    check_reference_as_mapped_type<std::unordered_multimap>();
 }