Why std::is_constructible stops at immediate context?

Originated from this topic. Also related to this topic.

My question is why std::is_constructible stops at the immediate context? I think users of std::is_constructible would expect it to work in full depth and give an exact answer. With this immediate context thing, you may have std::is_constructible give you a green light, only to get a hard compiler error when you actually do it. Doesn't this defeat the original goal and purpose of std::is_constructible. Now, it basically looks useless to me. I guess std::looks_constructible_at_first_sight would be a better name for the current semantics :(