[DNM] Implicitly open existentials by default #41328
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When calling a generic function with an argument of existential type,
implicitly "open" the existential type into a concrete archetype, which
can then be bound to the generic type. This extends the implicit
opening that is performed when accessing a member of an existential
type from the "self" parameter to all parameters. For example:
func unsafeFirst<C: Collection>(_ c: C) -> C.Element { c.first! }
func g(c: any Collection) {
unsafeFirst(c) // currently an error
// with this change, succeeds and produces an 'Any'
}
This avoids many common sources of errors of the form
protocol 'P' as a type cannot conform to the protocol itself
which come from calling generic functions with an existential, and
allows another way "out" if one has an existention and needs to treat
it generically.
This feature is behind a frontend flag
`-enable-experimental-opened-existential-types`.
An opaque type is only invariant with respect to the existential Self
when the constraints on the opaque type involve Self. Such constraints
are not expressible in the type-erased value, so treat them as
invariant. This loosens the restriction on using members of protocol
type that return an opaque type, such that (e.g.) the following is
still well-formed:
protocol P { }
protocol Q { }
extension P {
func getQ() -> some Q { ... }
}
func test(p: any P) {
let q = p.getQ() // formerly an error, now returns an "any Q"
}
However, this does not permit uses of members such as:
extension P {
func getCollection() -> some Collection<Self> { ... } // error
}
because the type system cannot express the corresponding existential
type `any Collection<Self>`.
Ensure that we only open existentials in an argument when the corresponding parameter's type is a generic parameter that is only used in covariant positions, because either invariant or contravariant uses mean that we won't be able to type-erase other uses of that parameter. This mirrors the requirement placed when opening existentials as a member of protocol type.
C++ function templates require specialization, which does not work with opened existentials. Disable opening for them.
…entials Implicitly disable implicit opening of existentials for SwiftOnoneSupport because it generates different specializations.
As with many other places in the frontend, grab the constraint type when from an existential type when we have one.
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@swift-ci please test source compatibility |
DougGregor
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Test PR to try out implicitly opening existentials by default