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Docs: Reattach, rewrite and rename the «associated-type-requirements»…
… educational note
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AnthonyLatsis committed Aug 30, 2021
1 parent 2800464 commit ccaa6707fea2c6d8cd45649af68729b2b227c368
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// EDUCATIONAL_NOTES(DIAG_ID, EDUCATIONAL_NOTE_FILENAMES...)

EDUCATIONAL_NOTES(unsupported_existential_type,
"associated-type-requirements.md")
EDUCATIONAL_NOTES(could_not_use_member_on_existential,
"existential-member-access-limitations.md")

EDUCATIONAL_NOTES(cannot_pass_type_to_non_ephemeral, "temporary-pointers.md")
EDUCATIONAL_NOTES(cannot_pass_type_to_non_ephemeral_warning,

This file was deleted.

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# Using Protocol Members with References to `Self` or `Self`-rooted Associated Types

Protocol requirements and protocol extension members may be accessed via a conformance constraint on a generic parameter, an opaque result type, or via the protocol type itself:

```swift
// An appropriately constrained generic parameter.
func foo<T: CustomStringConvertible>(arg: T) {
let description: String = arg.description
}
do {
// An appropriately constrained opaque result type.
func foo() -> some CustomStringConvertible { true }
let description: String = foo().description
}
// The protocol type.
func foo(arg: CustomStringConvertible) {
let description: String = arg.description
}
```

While the former two options enable full access to the protocol interface, not all members may be accessible when the protocol is used as a type and not a constraint. Specifically, a protocol member cannot be accessed on a protocol type when its type signature contains a reference to `Self` or a `Self`-rooted associated type. Accessing such members on a protocol type is not supported because today the compiler does not have a well-defined meaning and means of representation for `Self` and `Self`-rooted associated types with respect to a protocol type `P`. As a result, the following code is not allowed:

```swift
protocol Shape {
func matches(_ other: Self) -> Bool
}
func foo(_ shape: Shape) {
// error: member 'matches' cannot be used on value of protocol type 'Shape'; use a generic constraint instead
shape.matches(shape)
}
func foo(_ arg: Identifiable) {
// error: member 'id' cannot be used on value of protocol type 'Identifiable'; use a generic constraint instead
_ = arg.id
}
```

An exception to this limitation are members that contain `Self` only in [covariant](https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)) position (such as a method result type), where `Self` can be safely substituted with the protocol or protocol composition type used to access the member — a representable supertype. On the other hand, resorting to this ploy in contravariant parameter type position, like allowing one to pass a type-erased value to a method that accepts `Self`, is not type-safe and would expose the opportunity to pass in an argument of non-matching type.

```swift
protocol Shape {
func duplicate() -> Self
}
func duplicateShape(_ shape: Shape) -> Shape {
return shape.duplicate // OK, produces a value of type 'Shape'
}
```

Most use cases involving usage of protocol members that fall under the above restriction can instead be supported by constrained generics, opaque result types, or manual type-erasing wrappers. To learn more, see the sections on [protocols](https://docs.swift.org/swift-book/LanguageGuide/Protocols.html), [generics](https://docs.swift.org/swift-book/LanguageGuide/Generics.html), and [opaque types](https://docs.swift.org/swift-book/LanguageGuide/OpaqueTypes.html) in the Language Guide. For a better understanding of existential types in particular, and an in-depth exploration of the relationships among these built-in abstraction models, we recommend reading the [design document for improving the UI of the generics model](https://forums.swift.org/t/improving-the-ui-of-generics/22814).

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