diff --git a/libcxx/include/map b/libcxx/include/map
index 16127a5d76bc..f533d8f4b8a9 100644
--- a/libcxx/include/map
+++ b/libcxx/include/map
@@ -835,7 +835,15 @@ public:
     typedef _VSTD::reverse_iterator<const_iterator>         const_reverse_iterator;
 
     _LIBCPP_INLINE_VISIBILITY
-    map(const key_compare& __comp = key_compare())
+    map()
+        _NOEXCEPT_(
+            is_nothrow_default_constructible<allocator_type>::value &&
+            is_nothrow_default_constructible<key_compare>::value &&
+            is_nothrow_copy_constructible<key_compare>::value)
+        : __tree_(__vc(key_compare())) {}
+
+    _LIBCPP_INLINE_VISIBILITY
+    explicit map(const key_compare& __comp)
         _NOEXCEPT_(
             is_nothrow_default_constructible<allocator_type>::value &&
             is_nothrow_default_constructible<key_compare>::value &&
@@ -1568,7 +1576,15 @@ public:
     typedef _VSTD::reverse_iterator<const_iterator>         const_reverse_iterator;
 
     _LIBCPP_INLINE_VISIBILITY
-    multimap(const key_compare& __comp = key_compare())
+    multimap()
+        _NOEXCEPT_(
+            is_nothrow_default_constructible<allocator_type>::value &&
+            is_nothrow_default_constructible<key_compare>::value &&
+            is_nothrow_copy_constructible<key_compare>::value)
+        : __tree_(__vc(key_compare())) {}
+
+    _LIBCPP_INLINE_VISIBILITY
+    explicit multimap(const key_compare& __comp)
         _NOEXCEPT_(
             is_nothrow_default_constructible<allocator_type>::value &&
             is_nothrow_default_constructible<key_compare>::value &&
diff --git a/libcxx/include/set b/libcxx/include/set
index f093b1896a30..7dbf97095e24 100644
--- a/libcxx/include/set
+++ b/libcxx/include/set
@@ -425,12 +425,21 @@ public:
     typedef _VSTD::reverse_iterator<const_iterator> const_reverse_iterator;
 
     _LIBCPP_INLINE_VISIBILITY
-    set(const value_compare& __comp = value_compare())
+    set()
+        _NOEXCEPT_(
+            is_nothrow_default_constructible<allocator_type>::value &&
+            is_nothrow_default_constructible<key_compare>::value &&
+            is_nothrow_copy_constructible<key_compare>::value)
+        : __tree_(value_compare()) {}
+
+    _LIBCPP_INLINE_VISIBILITY
+    explicit set(const value_compare& __comp)
         _NOEXCEPT_(
             is_nothrow_default_constructible<allocator_type>::value &&
             is_nothrow_default_constructible<key_compare>::value &&
             is_nothrow_copy_constructible<key_compare>::value)
         : __tree_(__comp) {}
+
     _LIBCPP_INLINE_VISIBILITY
     explicit set(const value_compare& __comp, const allocator_type& __a)
         : __tree_(__comp, __a) {}
@@ -822,12 +831,21 @@ public:
 
     // construct/copy/destroy:
     _LIBCPP_INLINE_VISIBILITY
-    multiset(const value_compare& __comp = value_compare())
+    multiset()
+        _NOEXCEPT_(
+            is_nothrow_default_constructible<allocator_type>::value &&
+            is_nothrow_default_constructible<key_compare>::value &&
+            is_nothrow_copy_constructible<key_compare>::value)
+        : __tree_(value_compare()) {}
+
+    _LIBCPP_INLINE_VISIBILITY
+    explicit multiset(const value_compare& __comp)
         _NOEXCEPT_(
             is_nothrow_default_constructible<allocator_type>::value &&
             is_nothrow_default_constructible<key_compare>::value &&
             is_nothrow_copy_constructible<key_compare>::value)
         : __tree_(__comp) {}
+
     _LIBCPP_INLINE_VISIBILITY
     explicit multiset(const value_compare& __comp, const allocator_type& __a)
         : __tree_(__comp, __a) {}