[SR-14369] Pattern matching allows implicit (de)labeling on enum payloads.

[swift-ci]

Previous ID	SR-14369
Radar	rdar://problem/75536455
Original Reporter	ensan (JIRA User)
Type	Bug

Environment

Apple Swift version 5.3.2 (swiftlang-1200.0.45 clang-1200.0.32.28)

Target: x86_64-apple-darwin20.3.0

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 113d93ddba44a19aed68abb4e5d47b5d

Issue Description:

Consider following enum.

enum Either<T,S> {
    case first(T)
    case second(S)
}

This is normal usage:

let value = Either<Int, String>.first(42)
switch value{
case let .first(value):
    print(value)
case let .second(value):
    print(value)
}

However following also work. Obviously it is strange because enum doesn't have any labels.

switch value{
case let .first(aaaa: int):
    print(int)
case let .second(bbbb: string):
    print(string)
}

It's also happen with `if` and other statements with `case`.

if case let .first(aaaa: int) = value{
}

I don't know deeply about swift compiler, but it seems to be a bug of compiler.

[typesanitizer]

Swift does have some deliberate support for implicit conversions between tuples with labels and without labels. I suspect this snuck in because the implementation for enum payloads in patterns uses tuples, which means we get the conversions "for free". So I'm not sure if this is deliberate or "undesigned behavior". Seems unintentional...

[typesanitizer]

@swift-ci create

[typesanitizer]

Also, we risk breaking people's code that is accidentally relying on this...