cc @rudkx, @xedin. It seems to me this has a legitimate reason for compiling previously, and still should be able to compile using `compactMap`. Working backward from the contextual result type `[Bar]`, we should be able to infer `[Bar?]` for the array literal type, which is a supertype of all of the elements of the array literal.

@swift-ci create

Does it also work if you use $0 as Bar? to give explicit type information to the closure body?

It does compile with that in the closure body, that's the last example above. It also compiles with this:

let bars: [Bar] = [
    self.a as Bar?,
    self.b as Bar?,
].compactMap { $0 }

Sorry, I read that you had as? Bar in the closure body in your last example, which is slightly different from as Bar?.

Ah sorry I misread that! No that fails with this error:

bar.swift:15:27: error: 'Superclass?' is not convertible to 'Bar?'; did you mean to use 'as!' to force downcast?
        ].compactMap { $0 as Bar? }
                       ~~~^~~~~~~
                          as!

@rudkx It seems like we do something very wrong here since with `$0 as? Bar` the result of the closure is inferred to be `Superclass` and `$0` to be `Superclass?` instead of `Bar` and `Bar?`. I'm going to take a look, it might be an easy fix.

@swift-ci create

The problem should be resolved by #15009 @keith, thanks for reporting and you can verify the fix by using next available nightly snapshot.

@xedin I tested the master snapshot from 3/8 and it does compile now. Thanks! What's the process for getting that fixed in the 4.1 branch?