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[SR-13707] Fix-it for missing let/var in ill-typed expression pattern #56104

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kavon opened this issue Oct 8, 2020 · 1 comment
Open

[SR-13707] Fix-it for missing let/var in ill-typed expression pattern #56104

kavon opened this issue Oct 8, 2020 · 1 comment
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compiler The Swift compiler in itself diagnostics QoI Bug: Diagnostics Quality of Implementation improvement type checker Area → compiler: Semantic analysis

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@kavon
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kavon commented Oct 8, 2020

Previous ID SR-13707
Radar rdar://problem/70075207
Original Reporter @kavon
Type Improvement
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Improvement, DiagnosticsQoI, TypeChecker
Assignee None
Priority Medium

md5: 56d9c5d34022ff4984c7e5bdb640d5e2

relates to:

  • SR-13706 Fix-it for missing let/var in pattern when identifier is unbound

Issue Description:

Consider this incorrect code:

enum List {
    indirect case Cons(Int, List)
    case Nil
}

func hd(_ lst : List) -> Int? {
    switch lst {
        case .Cons(hd, _): return hd
        case .Nil: return nil
    }
}

let lst = List.Cons(1, List.Nil)
if let h = hd(lst) {
    print(h)
}

The programmer (me) thought they were introducing a new binding, `hd` in the Cons case pattern. But, that requires a let/var in the pattern, so the compiler interprets it as an expression pattern. Not to be confused with SR-13706, here `hd` is bound as the name of the function itself, so we get the following type error:

missing_let_int.swift:9:20: error: expression pattern of type '(List) -> Int?' cannot match values of type 'Int'
        case .Cons(hd, _): return hd
                   ^~
missing_let_int.swift:9:35: error: cannot convert return expression of type '(List) -> Int?' to return type 'Int?'
        case .Cons(hd, _): return hd
                                  ^~

In this case, a fix-it may be warranted to suggest that they add a let/var in the pattern. The errors can get quite nasty and confusing when making this mistake for a more complex enum type, such as in this code:

enum List<T> {
    indirect case Cons(T, List<T>)
    case Nil
}

func hd<T>(_ lst : List<T>) -> T? {
    switch lst {
        case .Cons(hd, _): return hd
        case .Nil: return nil
    }
}

let lst = List.Cons(1, List.Nil)
if let h = hd(lst) {
    print(h)
}

Which gives all of these errors:

missing_let.swift:9:20: error: expression pattern of type '(List<T>) -> T?' cannot match values of type 'T'
        case .Cons(hd, _): return hd
                   ^~
missing_let.swift:9:20: error: generic parameter 'T' could not be inferred
        case .Cons(hd, _): return hd
                   ^
missing_let.swift:7:6: note: in call to function 'hd'
func hd<T>(_ lst : List<T>) -> T? {
     ^
missing_let.swift:9:35: error: cannot convert return expression of type '(List<T>) -> T?' to return type 'T?'
        case .Cons(hd, _): return hd
                                  ^~
                                     as! T
missing_let.swift:9:35: error: generic parameter 'T' could not be inferred
        case .Cons(hd, _): return hd
                                  ^
missing_let.swift:7:6: note: in call to function 'hd'
func hd<T>(_ lst : List<T>) -> T? {
     ^

theindigamer (JIRA User) agrees that a fix-it may be good here.
@typesanitizer
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@swift-ci create

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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compiler The Swift compiler in itself diagnostics QoI Bug: Diagnostics Quality of Implementation improvement type checker Area → compiler: Semantic analysis
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@kavon @typesanitizer