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[SR-6955] Pattern matching with generic-typed values can't do implicit casting #49503

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hamishknight opened this issue Feb 8, 2018 · 4 comments
Open

[SR-6955] Pattern matching with generic-typed values can't do implicit casting #49503

hamishknight opened this issue Feb 8, 2018 · 4 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself

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@hamishknight
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Previous ID SR-6955
Radar None
Original Reporter @hamishknight
Type Bug
Environment

Swift version 4.1-dev (LLVM b1f1b1f5b8, Clang d8b11579e8, Swift a455db6)
Target: x86_64-apple-darwin17.3.0

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

md5: aecd7e6fe6ed6d3cd2c53f25c945b2e6

Issue Description:

The following fails to compile:

enum E {
  case a
}

func foo<T>(_ t: T) {
  switch t {
  case E.a: // error: Enum case 'a' is not a member of type 'T'
    print("is E.a")
  default:
    print("isn't E.a")
  }
}

However it does compile if the value being switched over is an existential:

enum E {
  case a
}

func foo(_ t: Any) {
  switch t {
  case E.a:
    print("is E.a")
  default:
    print("isn't E.a")
  }
}

Unless there's a good reason not to allow this, IMO the logic in TypeCheckPattern.cpp should check for archetypes as well as existentials.
@hamishknight
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This is especially non-obvious in places where users can't see the generic placeholder, for example this compiles:

enum E : Error {
  case a
}

var e: Error = E.a
switch e {
case E.a:
  print("is E.a")
default:
  print("isn't E.a")
}

but this doesn't:

extension Error {
  func foo() {
    switch self {
    case E.a: // error: Enum case 'a' is not a member of type 'Self'
      print("is E.a")
    default:
      print("isn't E.a")
    }
  }
}

var e: Error = E.a
e.foo()

@belkadan
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belkadan commented Feb 8, 2018

Makes sense to me. @jckarter, any opinions?

@jckarter
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jckarter commented Feb 8, 2018

I agree this should work.

@hamishknight
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I've opened a PR here: #14496 @jckarter would you be available to review it?

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself
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@jckarter @belkadan @hamishknight