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[SR-11463] Type checker failure when bundling generics in a protocol #53863

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mbrandonw opened this issue Sep 12, 2019 · 5 comments
Closed

[SR-11463] Type checker failure when bundling generics in a protocol #53863

mbrandonw opened this issue Sep 12, 2019 · 5 comments
Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself generics Feature: generic declarations and types type checker Area → compiler: Semantic analysis

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@mbrandonw
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Previous ID SR-11463
Radar None
Original Reporter @mbrandonw
Type Bug
Status Closed
Resolution Invalid
Environment

Xcode 11, beta 6

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, Generics, TypeChecker
Assignee None
Priority Medium

md5: dba09fb15865679b69759637aed6b1f2

Issue Description:

The following gives a type checker error:

protocol TypeBundle {
    associatedtype A
    associatedtype B
}

typealias Update<T: TypeBundle> = (T.A, T.B) -> T.A
typealias InoutUpdate<T: TypeBundle> = (inout T.A, T.B) -> Void

func toInout<T: TypeBundle>(_ update: @escaping Update<T>) -> InoutUpdate<T> {
    fatalError()
}

enum MyTypes: TypeBundle {
    typealias A = Int
    typealias B = ()
}

var update: Update<MyTypes>!

toInout(update)
// Cannot convert value of type 'Update<MyTypes>?' (aka 'Optional<(Int, ()) -> Int>') to expected argument type '(_.A, _.B) -> _.A'

If I manually unroll the typealiases I can get it to type check:

protocol TypeBundle {
    associatedtype A
    associatedtype B
}

typealias Update<T: TypeBundle> = (T.A, T.B) -> T.A
typealias InoutUpdate<T: TypeBundle> = (inout T.A, T.B) -> Void

func toInout<A, B>(_ update: @escaping (A, B) -> A) -> (inout A, B) -> Void {
    fatalError()
}

enum MyTypes: TypeBundle {
    typealias A = Int
    typealias B = ()
}

var update: Update<MyTypes>!

toInout(update)
@theblixguy
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cc @xedin

@belkadan
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The error message is bad, but it's not really surprising the type-checker fails: there's no way to rederive MyTypes from Update<MyTypes>, because a typealias is just an alias. And that means the <T: TypeBundle> part of toInout can't be derived from the call site.

@mbrandonw
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@belkadan yep thanks! we came to that conclusion after grappling with it for a bit. was mysterious at first.

@belkadan
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We could still keep the bug around to improve the error message. We have a "cannot deduce generic parameter 'Blah'" that we use elsewhere, and I'm not sure why it's not kicking in here.

@xedin
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xedin commented Sep 20, 2019

Just FYI after argument-to-parameter mismatch has landed this diagnostic has been improved:

```
error: generic parameter 'T' could not be inferred
toInout(update)
^
note: in call to function 'toInout'
func toInout<T: TypeBundle>(_ update: @escaping Update<T>) -> InoutUpdate<T> {
^
```

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself generics Feature: generic declarations and types type checker Area → compiler: Semantic analysis
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@xedin @mbrandonw @belkadan @theblixguy