Is it illegal to invoke a std::function under the standard?

Question

All quotes are from N3797.

4/3 [conv]

An expression e can be implicitly converted to a type T if and only if the

Answer 1

Yes, your analysis is correct; I came to the same conclusion here.

According to Daniel Kruegler, this issue should appear on the library defect list subsequent to the next mailing:

A corresponding library issue has already been submitted, but is not yet visible in the issue list.

Hopefully once that becomes visible we'll also have a conclusive answer to whether it is allowable to construct a std::function with signature returning void passing a callable with signature returning non-void (Using `std::function` to call non-void function).

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Update: this was entered as LWG 2420, which was resolved in favor of special-casing void return type to static_cast the result of the invoked function to void. This means that a callable returning non-void can be the target of a std::function. LWG2420 was applied as a post-publication correction to C++14; meanwhile, all compilers I'm aware of effectively apply this behavior as an extension in C++11 mode.