The SIL for the X.add curry thunk is wrong – there's a spurious `copy_addr` in there:

  // function_ref X.add(_:)
  %1 = function_ref @$S2xx1XV3addyySiF : $@convention(method) (Int, @inout X) -> () // user: %4
  %2 = alloc_stack $X                             // users: %5, %4, %3
  copy_addr %0 to [initialization] %2 : $*X       // id: %3
  %4 = partial_apply [callee_guaranteed] %1(%2) : $@convention(method) (Int, @inout X) -> () // user: %6
  dealloc_stack %2 : $*X                          // id: %5
  return %4 : $@callee_guaranteed (Int) -> ()     // id: %6

I think this is a regression from the +0 arguments work.

Comment by Vatsal Manot (JIRA)

@slavapestov is there a workaround for this? I've been using this in a generic context:

@inlinable public func build<T, U, V>(_ x: T, with f: ((inout T) throws -> ((U) -> V)), _ y: U) rethrows -> T
{
    var x = x
    
    _ =  try f(&x)(y)
    
    return x
}

So using an unbound method reference like X.add in this case is not a good idea if the method is mutating, because you get undefined behavior if the method reference is allowed to escape.

In this example, you're immediately applying it twice, so we could make it work again.

As a workaround, just write an explicit closure instead of referencing X.add.

You can change build() to take a (inout T, U) throws -> V instead of a ((inout T) throws -> ((U) -> V)), and then pass in { $0.add($1) } instead of X.add.

Comment by Vatsal Manot (JIRA)

@slavapestov I've been using this to shorten:

var x = self
x.mutatingFunc()
return x

... in about 78 callsites.

Comment by Vatsal Manot (JIRA)

Ah... I see. Thank you for the prompt response!

@swift-ci create

I don’t think we can reasonably make this work at all with inout exclusivity enforcement, so we’re going to have to break compatibility here. At one point we had code that banned partial application of mutating methods, but that must have either broke, or there’s a loophole. We should figure out why it isn’t getting diagnosed.

Here's a patch that warns in this case (upgraded to an error in Swift 5 mode): #17642

Merged. You should get a warning now that this isn't really supported.