Improve error message for type mismatch between any P and (any P)?

[hborla]

If you try to pass a value of type (any P)? to an argument of type any P, Swift should produce an error telling you to unwrap the optional. Instead, it tells you that (any P)? doesn’t conform to P, which is confusing, and the user probably meant to unwrap the argument anyway.

protocol P {}

struct S: P {}

func takesP(_: any P) {}

func passOptional(value: (any P)?) {
  takesP(value) // error: Argument type '(any P)?' does not conform to expected type 'P'

  takesP(value ?? S()) // okay
  takesP(value!) // okay
}

The constraint system currently applies the MissingConformance fix when solving for the takesP(value) expression. Instead, it should identify that the wrapped type of the argument matches the parameter type, and apply the ForceOptional fix.

This is also tracked by rdar://94037733

[hborla]

cc @dfperry5 this is a good constraint system starter bug if you're interested!

[dfperry5]

@hborla - I am so interested and am happy to take it on! Can you assign it to me? For some reason I can't see the "assign yourself" button.

[dfperry5]

Opened PR here: #61191