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[SR-5198] Diagnostics are pretty much always useless when errors involve type inference and closures #47774

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NachoSoto opened this issue Jun 12, 2017 · 15 comments
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[SR-5198] Diagnostics are pretty much always useless when errors involve type inference and closures #47774

NachoSoto opened this issue Jun 12, 2017 · 15 comments
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@NachoSoto
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself diagnostics QoI Bug: Diagnostics Quality of Implementation type checker Area → compiler: Semantic analysis

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@NachoSoto
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Previous ID SR-5198
Radar rdar://problem/33914448
Original Reporter @NachoSoto
Type Bug
Status Resolved
Resolution Done
Environment

  • Swift version 4.0 (swiftlang-900.0.43 clang-900.0.22.8)

  • Swift version 4.0 (swiftlang-900.0.63.10 clang-900.0.36)

  • Swift version 4.0.2 (swiftlang-900.0.69.1 clang-900.0.38)

  • Swift version 4.0.3 (swiftlang-900.0.74.1 clang-900.0.39.2)

  • Swift version 4.1 (swiftlang-902.0.34 clang-902.0.30)

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, DiagnosticsQoI, TypeChecker
Assignee @NachoSoto
Priority Medium

md5: 6266990ac5135a6213094a3641402482

Issue Description:

I'm filing a particular example, but this is just one of the hundreds I've ran into regarding this since Swift 1.0. I honestly don't think this has gotten any better since the very first version.

This is a reduction from my own codebase. As I was going through the Swift 4.0 migrations, lots of places relying on tuple deconstruction in closure parameters stopped working, and diagnostics leave A LOT to be desired (https://twitter.com/NachoSoto/status/874336197568454656, https://twitter.com/NachoSoto/status/874336844757975040).

Consider this:

protocol P {
    associatedtype E    
}

struct S<A> {
    init() {}   
}

struct B<Y>: P {
    typealias E = Y
}

class X {
    func f1() -> S<Void> {
        return f(
            d: { _ in () }, 
            b: B(), 
            isRepeat: { _, _ in false }
        )
    }

    private func f<A, B: P>(
        d: @escaping (String) throws -> A, 
        b: B?, 
        isRepeat: @escaping (A, A) -> Bool
    ) -> S<A> where B.E == A {
        return S()
    }
}

This was compiling in Swift 3.x. Notice the `isRepeat` parameter I'm passing only has one _.

error: cannot convert value of type '()' to closure result type '_.E'
        d: { _ in () },

Say what? What's the problem in d? That looks correct to me...
Spoiler alert, the correct fix is changing isRepeat to

_, _

Instead of just _.

Again, like I said in the title, this is not an isolated bug, this happens ALL THE TIME when types can't be inferred. Diagnostic always point to the wrong thing with a useless message.

When are you going to start focusing on diagnostics? I have filed SO MANY diagnostic bugs that are still open:
@belkadan
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cc @xedin

@NachoSoto
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Another example showing how Swift fails to produce diagnostics in the most simple cases: SR-5233

@NachoSoto
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Another really simple example @belkadan @xedin... SR-5243
I'm gonna keep filing these as I find them, hoping that one day getting things wrong will actually make Swift tell you exactly what's wrong.

@NachoSoto
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And another one: https://bugs.swift.org/browse/SR-5245

@NachoSoto
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FYI I've been updating the list of diagnostic issues here. It's grown to 14 including this one... I hope you guys can get to them soon cause the state of this in Swift 4.0 is currently pretty embarrassing.

@slavapestov
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@swift-ci create

@NachoSoto
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Any update on these? I just filed SR-6285, as I keep running into more and more useless diagnostics. Honestly, nobody cares about ABI stability if we can't actually write code.

@NachoSoto
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And another one: SR-6292.

@NachoSoto
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Is there going to be any focus on diagnostics for any future version of Swift or should I just give up filing these?

@xedin
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xedin commented Jan 17, 2018

@NachoSoto Please keep filling the bugs! The problem is that, as I mentioned, the framework has to change completely before we can properly diagnose certain cases, I'm working on it, that just takes time to do.

@NachoSoto
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Is this going to be a priority for Swift 5.0?

@NachoSoto
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Added another one: SR-6876. Is this going to be a priority for Swift 5.0? I'm up to 17 unresolved diagnostic Jiras...

@xedin
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xedin commented Jan 30, 2018

@NachoSoto Thanks! Diagnostics are always a priority, but fixing them usually requires significant time, that's why everything is going slowly.

@NachoSoto
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Another one: SR-6994.

@xedin
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xedin commented Feb 5, 2020

I have removed `, _` from `isRepeat` and tried with 5.2 snapshot and got following diagnostic:

error: contextual closure type '(Void, Void) -> Bool' expects 2 arguments, but 1 was used in closure body
            isRepeat: { _ in false }
                        ^
                         ,_

This is exactly what you are taking about so I'd really appreciate if you have verified using 5.2 nightly and closed. I'll take a look at the rest of the list meanwhile.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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@NachoSoto NachoSoto

Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself diagnostics QoI Bug: Diagnostics Quality of Implementation type checker Area → compiler: Semantic analysis
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@xedin @slavapestov @NachoSoto @belkadan