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[SR-922] Inconsistent handling of labeled and unlabeled tuples #43534

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natecook1000 opened this issue Mar 11, 2016 · 5 comments
Closed

[SR-922] Inconsistent handling of labeled and unlabeled tuples #43534

natecook1000 opened this issue Mar 11, 2016 · 5 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself type checker Area → compiler: Semantic analysis

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@natecook1000
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Previous ID SR-922
Radar None
Original Reporter @natecook1000
Type Bug
Status Resolved
Resolution Won't Do
Environment

Latest Swift version 3.0-dev

Additional Detail from JIRA
Votes 1
Component/s Compiler
Labels Bug, TypeChecker
Assignee None
Priority Medium

md5: 522299af37596ed9074109d223562b2e

Issue Description:

Swift 3's type checker sometimes doesn't match tuples that have the right type but are missing labels for the values. This seems like a regression from Swift 2.2:

struct MyDictionary<Key: Hashable, Value> { 
  typealias KeyValueTuple = (key: Key, value: Value) 
   
#if swift(>=3.0)
  func foo<S: Sequence where S.Iterator.Element == KeyValueTuple>(s: S) {
    print(s) 
  } 
#else
  func foo<S: SequenceType where S.Generator.Element == KeyValueTuple>(s: S) {
    print(s) 
  } 
#endif
} 

let labeled = (key: "a", value: 1)
let unlabeled = ("a", 1)

let dictionary = MyDictionary<String, Int>()

// These work in Swift 2.2 and 3.0
dictionary.foo([(key: "a", value: 1)])
dictionary.foo([("a", 1)])
dictionary.foo([labeled])

// This fails in Swift 3.0
dictionary.foo([unlabeled])
@belkadan
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cc cwillmore (JIRA User)

@swift-ci
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Comment by Dan Appel (JIRA)

This issue requires code to have explicitly labeled tuples all around. For example, the following code (in Swift 2)

extension CollectionType where Generator.Element == (String, Int) {}

Would match both a dictionary `[String:Int]` and an array of tuples `[(String, Int)]` (which is essentially a dictionary).

However, in Swift 3, the tuples are required to be labeled, otherwise the extensions don't apply.

extension Collection where Iterator.Element == (key: String, value: Int) {}

Which does match dictionaries `[String:Int]` and arrays `[(key: String, value: Int)]` but not the arrays without the labels `[(String, Int)]`.

As you can imagine, this is a major inconvenience and seems to give labeled tuples a separate type, even though in both cases their values can still be accessed with `tuple.0` and `tuple.1`.

@slavapestov
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This is working as intended – labeled and unlabeled tuple types are different for purposes of same-type constraints, even though they can convert to each other in expressions.

@swift-ci
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swift-ci commented Jun 3, 2017

Comment by Tim (JIRA)

FWIW, this issue is causing the new Swift 4 methods Dictionary.merge and Dictionary.merging to not accept other dictionaries.

@rudkx
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rudkx commented Jun 3, 2017

The issues with Dictionary.merge and Dictionary.merging are covered in https://bugs.swift.org/browse/SR-4969.

@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 type checker Area → compiler: Semantic analysis
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@slavapestov @rudkx @belkadan @natecook1000 @swift-ci