cpp/types/is constructible

@1@ If is an object or reference type and the variable definition  is well-formed, provides the member constant  equal to. In all other cases, is. For the purposes of this check, the variable definition is never interpreted as a function declaration, and the use of std is not considered an odr-use. Access checks are performed as if from a context unrelated to and any of the types in. Only the validity of the immediate context of the variable definition is considered.

@2@ Same as, but the variable definition does not call any operation that is not trivial. For the purposes of this check, the call to std is considered trivial.

@3@ Same as, but the variable definition is.