[swift-users] Swift's method dispatch

[Slava Pestov]

Hi Jens,

I think the problem is that overload ranking always prefers a protocol requirement to a protocol extension member, because usually you want the dynamic dispatch through the requirement instead of calling the default implementation. But it appears that this heuristic does not take into account the fact that the protocol extension member could be more constrained than the requirement.

Please file a bug, but it is unclear what the desired behavior actually is here. Perhaps it should just diagnose an ambiguity.

Slava

> On Dec 8, 2017, at 6:25 AM, Jens Persson via swift-users <swift-users at swift.org> wrote:
> 
> Hi all!
> 
> Can someone please explain the rationale behind the last line printing
> "T is unknown"
> rather than (what I would expect):
> "T is Int"
> in the following program?
> 
> 
> protocol P {
>     associatedtype T
>     func f() // *
> }
> extension P {
>     func f() { print("T is unknown") }
> }
> extension P where T == Int {
>     func f() { print("T is Int") }
> }
> 
> struct X<T> : P {}
> 
> struct Y<U> where U: P, U.T == Int {
>     // NOTE: The compiler/type-checker knows that U.T == Int here so ...
>     typealias T = U.T
>     var a: U
>     func g() { a.f() } // ... how/why could this print anything but "T is Int"?
> }
> 
> let x = X<Int>()
> x.f() // Prints "T is Int", no matter if * is commented out or not.
> 
> let y = Y(a: X<Int>())
> y.g() // Prints "T is unknown" unless * is commented out. Why?
> 
> 
> IMHO this looks like the compiler simply ignores that struct Y<U> has the constraint  U.T == Int.
> How else to explain this behavior?
> /Jens
> 
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