cpp/types/is convertible

@1@ If the imaginary function definition is well-formed, (that is, either  can be converted to  using implicit conversions, or both  and  are possibly cv-qualified ), provides the member constant  equal to. Otherwise is. For the purposes of this check, the use of std in the return statement is not considered an odr-use. @@ Access checks are performed as if from a context unrelated to either type. Only the validity of the immediate context of the expression in the return statement (including conversions to the return type) is considered. @2@ Same as, but the conversion is also.