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@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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