N3727 here: http://open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3727.html

In my view there is an important thing missing from this
specification: text that states that those overloads do not
participate on overload resolution unless the corresponding relevant
expression is well-formed.

The first overload, with result_of, already has that behaviour since
result_of lacks a member type when that's the case, so it's actually
ok.

But the second overload will always take part in overload resolution,
even if the relevant call expression is not well-formed. I think this
is unfortunate and requires jumping through hoops if you are making a
decision on whether some type is "invokable with a given result type".

Mit freundlichen Gr=FC=DFen,

Martinho

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