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[SR-10852] [Opaque Result Types] Generic parameter 'τ_0_0' could not be inferred #53242

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theblixguy opened this issue Jun 7, 2019 · 10 comments
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[SR-10852] [Opaque Result Types] Generic parameter 'τ_0_0' could not be inferred #53242

theblixguy opened this issue Jun 7, 2019 · 10 comments
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@theblixguy
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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 opaque types Feature → types: opaque types operators Feature: operators swift 5.1 type checker Area → compiler: Semantic analysis

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@theblixguy
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Previous ID SR-10852
Radar rdar://problem/51526846
Original Reporter @theblixguy
Type Bug
Status Resolved
Resolution Done
Environment

Xcode 11 Beta 1, Swift 5.1, macOS 10.15 Developer Beta 1 (19A471t)

Additional Detail from JIRA
Votes 1
Component/s Compiler
Labels Bug, DiagnosticsQoI, OpaqueResultTypes
Assignee @theblixguy
Priority Medium

md5: f781c969fc89c0f73616e7ae7b35dbfa

Issue Description:

I don't think "τ_{depth}_{index}" is supposed to leak into diagnostics:

protocol EquatableBox {
  associatedtype Value: Equatable
  var value: Value { get }
}

func ==<T: EquatableBox, U: EquatableBox>(lhs: T, rhs: U) -> Bool where T.Value == Int, U.Value == Int {
  return lhs.value == rhs.value
}

struct ConcreteEquatableBox: EquatableBox {
  typealias Value = Int
  let value: Value
}

func someEquatableBox() -> some EquatableBox {
  return ConcreteEquatableBox(value: 5)
}

let a = someEquatableBox()
let b = someEquatableBox()

let _ = a == b // Generic parameter 'τ_0_0' could not be inferred
@theblixguy
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cc @jckarter

@jckarter
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jckarter commented Jun 7, 2019

I wonder if it's getting confused doing overload resolution on `==` and misattributing the type error.

@swift-ci
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swift-ci commented Jun 8, 2019

Comment by Tapan Thaker (JIRA)

Taking this bug.
Just got a diagnostics PR landed in the TrySwift conf, so this would be a nice place to contribute next.

@swift-ci
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Comment by Tapan Thaker (JIRA)

The compilation will pass if I change the above `==` overload to below.

func ==<T: EquatableBox, U: EquatableBox>(lhs: T, rhs: U) -> Bool  {
  return lhs.value == rhs.value
}

My thought process is that, constraint solver is somehow not attributing the correct `==` overload with the where constraint.

@swift-ci
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Comment by Tapan Thaker (JIRA)

@jckarter: To me, it looks like ConstraintSolver solved and inferred the correct types. Also, I get that the compiler could not find a
== overloading for some EquatableBox. Just not sure what diagnostic should now be emitted to make the problem obvious.

---Solution---
Fixed score: 2 0 0 0 0 0 0 0 0 0 0 0
Type variables:
  $T66 as some EquatableBox @ locator@0x7fa5430a3078 [OverloadedDeclRef@Hello.swift:26:11 -> generic parameter 'U']
  $T2 as some EquatableBox @ locator@0x7fa54389d7a0 [DeclRef@Hello.swift:26:14]
  $T1 as some EquatableBox @ locator@0x7fa54389d718 [DeclRef@Hello.swift:26:9]
  $T3 as Bool @ locator@0x7fa54389d830 [Binary@Hello.swift:26:11 -> function result]
  $T0 as (some EquatableBox, some EquatableBox) -> Bool @ locator@0x7fa54389a600 [OverloadedDeclRef@Hello.swift:26:11]
  $T65 as some EquatableBox @ locator@0x7fa5430a3030 [OverloadedDeclRef@Hello.swift:26:11 -> generic parameter 'T']

Overload choices:
  locator@0x7fa54389d7a0 [DeclRef@Hello.swift:26:14] with Hello.(file).b@Hello.swift:24:5 as b: some EquatableBox

  locator@0x7fa54389d718 [DeclRef@Hello.swift:26:9] with Hello.(file).a@Hello.swift:23:5 as a: some EquatableBox

  locator@0x7fa54389a600 [OverloadedDeclRef@Hello.swift:26:11] with Hello.(file).==@Hello.swift:6:6 as ==: ($T65, $T66) -> Bool


Constraint restrictions:

Disjunction choices:

Opened types:
  locator@0x7fa54389a600 [OverloadedDeclRef@Hello.swift:26:11] opens τ_0_0 -> $T65, τ_0_1 -> $T66

Fixes:
  [fix: skip same-type generic requirement] @ locator@0x7fa5430a3228 [OverloadedDeclRef@Hello.swift:26:11 -> opened generic -> type parameter requirement #&#8203;2 (same-type)]
  [fix: skip same-type generic requirement] @ locator@0x7fa5430a32d8 [OverloadedDeclRef@Hello.swift:26:11 -> opened generic -> type parameter requirement #&#8203;3 (same-type)]
---Constraint solving for the expression at [Hello.swift:26:9 - line:26:9]---
  (overload set choice binding $T0 := some EquatableBox)
---Initial constraints for the given expression---
(declref_expr type='some EquatableBox' location=Hello.swift:26:9 range=[Hello.swift:26:9 - line:26:9] decl=Hello.(file).a@Hello.swift:23:5 function_ref=unapplied)
Score: 0 0 0 0 0 0 0 0 0 0 0 0
Type Variables:
  $T0 [lvalue allowed] [noescape allowed] as some EquatableBox @ locator@0x7fa542960600 [DeclRef@Hello.swift:26:9]

Active Constraints:

Inactive Constraints:
Resolved overloads:
  selected overload set choice a: $T0 == some EquatableBox

  (found solution 0 0 0 0 0 0 0 0 0 0 0 0)
---Solver statistics---
Total number of scopes explored: 1
Maximum depth reached while exploring solutions: 1
Time: 1.239170e+02ms

@jckarter
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Thanks for looking into this! @xedin where would be a good place to improve the diagnostic here? The issue is that the constraints on
`where T.Value == Int, U.Value == Int` cannot be satisfied by `some EquatableBox` since the opaque type doesn't reveal the associated type of the underlying type. It seems like this would be a nice thing to provide diagnostics for in the general case of a generic parameter type that's missing constraints to be the generic argument to another call.

@xedin
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xedin commented Jun 15, 2019

tt (JIRA User) Looks like we already record fixes for unsatisfied requirements and form a solution so the only remaining problem to figure out is why don't we emit the diagnostics for recorded fixes.

Fixes:
  [fix: skip same-type generic requirement] @ locator@0x7fa5430a3228 [OverloadedDeclRef@Hello.swift:26:11 -> opened generic -> type parameter requirement #&#8203;2 (same-type)]
  [fix: skip same-type generic requirement] @ locator@0x7fa5430a32d8 [OverloadedDeclRef@Hello.swift:26:11 -> opened generic -> type parameter requirement #&#8203;3 (same-type)]

@xedin
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xedin commented May 8, 2020

Looks like we produce a much better error now on master:

error: operator function '==' requires the types '(some EquatableBox).Value' and 'Int' be equivalent
let _ = a == b
          ^

note: where 'T.Value' = '(some EquatableBox).Value'
func ==<T: EquatableBox, U: EquatableBox>(lhs: T, rhs: U) -> Bool where T.Value == Int, U.Value == Int {
     ^

WDYT, @theblixguy?

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
@Chevel
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Chevel commented Jan 19, 2023 •
edited

Follow up on this.
I am able to reproduce it with a much simpler example. Maybe it will offer some insight.

import SwiftUI
enum Fruit {

    case apple
    case orange

    var icon: (some View)? {
        switch self {
        case .apple: return Image(systemName: "applelogo")
        case .orange: return nil
        }
    }

}

@AnthonyLatsis AnthonyLatsis added type checker Area → compiler: Semantic analysis operators Feature: operators swift 5.1 labels Jan 20, 2023
@AnthonyLatsis
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AnthonyLatsis commented Jan 20, 2023 •
edited

I am able to reproduce it with a much simpler example.

@Chevel Yours appears to be a different bug, please open a new issue. This particular one was fixed.

@ios-blacksmith ios-blacksmith mentioned this issue Jan 21, 2023
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@theblixguy theblixguy

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 opaque types Feature → types: opaque types operators Feature: operators swift 5.1 type checker Area → compiler: Semantic analysis
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@xedin @jckarter @theblixguy @swift-ci @AnthonyLatsis @Chevel