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[SR-13917] Poor diagnostic when type inference infers non-intuitive generic parameter. #56315

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typesanitizer opened this issue Nov 29, 2020 · 4 comments
Open

[SR-13917] Poor diagnostic when type inference infers non-intuitive generic parameter. #56315

typesanitizer opened this issue Nov 29, 2020 · 4 comments
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compiler The Swift compiler in itself improvement type checker Area → compiler: Semantic analysis

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@typesanitizer
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Previous ID SR-13917
Radar rdar://problem/71912785
Original Reporter @typesanitizer
Type Improvement
Status Reopened
Resolution
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Improvement, TypeChecker
Assignee None
Priority Medium

md5: 5de0728de90f586ca372a09a1e33b463

Issue Description:

https://twitter.com/wilshipley/status/1332934549223862273

extension Set {
  func overlapCount(_ lhs: [Int], _ rhs: [Int]) -> Int {
    Set(lhs).intersection(rhs).count // error: type of expression is ambiguous without more context
  }
}

We should infer that T == Int here. If that's not possible, we should at least provide a more precise diagnostic.
@typesanitizer
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@swift-ci create

@xedin
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xedin commented Dec 8, 2020

Unfortunately this is existing behavior which we can't change due to source compatibility reasons. Usage of `Set` in extension of `Set` means that generic arguments are equated unless parameters are explicitly specified.

@typesanitizer
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Can we improve the diagnostic though? The "type of expression is ambiguous without more context" isn't very helpful.

@xedin
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xedin commented Dec 8, 2020

I don't really see how we could detect this in the constraint system since type variables would just get passed down to the references of Set, we can't allocate a fix per use...

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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compiler The Swift compiler in itself improvement type checker Area → compiler: Semantic analysis
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@xedin @typesanitizer