Allow open(f,filename,...) for f being any callable

[oxinabox]

Since Functions are not the only thing that is callable,
they should not be the only things allowed as first arguments.

Though one can always do

open(t -> mycallable(t), "temp.txt")

This broke me using ExpectationStubs.jl.

I don't believe this introduces an ambiguity, so it should be fine.

[Ismael-VC]

Couldn't we have another abstract type, supertype of Function called Callable or something like that?

Any -> Callable -> Function

[nalimilan]

Couldn't we have another abstract type, supertype of Function called Callable or something like that?

We could, but due to the lack of multiple inheritance some types might be unable to be marked as Callable, so that doesn't completely fix the problem.

[oxinabox]

My tests are going to hit #14747
if it is still occurring

[vtjnash]

Upstream branch seems to have been deleted. It doesn't seem critical (the OP already mentioned the workaround), and using ::Function seems likely to convey intent better than getting a MethodError later when trying to interpret the 2nd arg as a thing to open and the 1st arg as a thing to call