Remove no_parens when using capture with arity

[sabiwara]

Close #14089

To be backported

[sabiwara]

This is not strictly needed for the fix, but otherwise we're expanding to something like {:fn, [{:no_parens, true}], [{:->, [{:no_parens, true}], ... which feels weird and make it harder to test.

[josevalim]

I think ideally we want to compile this to:

Function.capture(expr, name, arity)

So something we get something like this back:

iex> &(if true, do: NaiveDateTime, else: DateTime).utc_now/0
&NaiveDateTime.utc_now/0

Note we already do this for variables:

iex> var = NaiveDateTime
iex> &var.utc_now/0
&NaiveDateTime.utc_now/0

So we probably need to generalize this branch of the code a bit more (i.e. it should apply whenever the left side of the . is an expression)?

[josevalim]

Nah, ignore me, there is clearly a semantic difference here. Your version executes foo() in &foo().bar/0 when the function is invoked. Mine would execute it always.

I think this is actually a bug. We should not have accepted this format but we clearly do. My suggestion is to deprecate this feature in main.

[josevalim]

To be clear beyond doubt:

&expr.fun/arity

Only works if:

• expr is a variable or an atom at compile-time
• fun is an atom at compile-time
• arity is an integer at compile-time

If those conditions are not met and it is valid, we should deprecate it.

[sabiwara]

Sounds good, I hesitated to propose something in this vein but was unsure if there would be other legit cases.
As you said assigning to a variable or using fn seems better either way.

[josevalim]

Ship it and please back port it.

I will open up a separate issue for the deprecation, feel free to tackle it!

[sabiwara]

Backported

I will open up a separate issue for the deprecation, feel free to tackle it!

Thanks, will do!