some docs

Author: vinay-deshmukh

libcxx/include/__tree

@@ -1277,7 +1277,13 @@ error: no matching constructor for initialization of '__get_node_value_type_t<va
       // , integral_constant<bool, is_copy_constructible_v<_From> >()
       // );
 
+      // TODO: narrow it to "_From.first"
       if constexpr (integral_constant<bool, is_copy_constructible_v<_From> >()) {
+        // we use copy, and not "move" as the constraint
+        // because we can NOT move from `const key_type`, which is how `value_type` is defined
+        // atleast for map
+        // typedef pair<const key_type, mapped_type> value_type;
+        // so we must copy it
 
       // const_cast is not allowed at constexpr time.
       // we get around this by deleting __lhs and creating a new node in-place
@@ -1299,7 +1305,7 @@ error: no matching constructor for initialization of '__get_node_value_type_t<va
 
       // (void)__lhs;
       // (void)__rhs;
-      // static_assert(false, "Can not move from a pair<const T, U>");
+      static_assert(false, "Can not move from a pair<const T, U>");
       // TODO: how do I flag this code to NOT run at compile time?
       // Using static_assert means this can't compile _at all_, regardless of whether this code is actually going to execute at compile time or not