Void-returning @differentiable function crashes compiler

[hamishknight]

The following crashes in SILGen:

import _Differentiation

func foo(_ fn: @differentiable(reverse) (Float) -> Void) {}

[asl]

So, the reason here is Void (aka empty tuple) type is assumed to be differentiable with tangent space being empty tuple as well.

Looks like we'd need to additionally check and disallow void result type if there are no inouts.

Tagging @rxwei @dan-zheng @BradLarson

[rxwei]

Yeah that sounds about right. @asl

[asl]

@rxwei Thanks for cross-checking! It tried to imagine other corner cases related to Void but failed to find any so far :)