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

[theblixguy]

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]

cc @jckarter

[jckarter]

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

[swift-ci]

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]

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]

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 #​2 (same-type)]
  [fix: skip same-type generic requirement] @ locator@0x7fa5430a32d8 [OverloadedDeclRef@Hello.swift:26:11 -> opened generic -> type parameter requirement #​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]

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]

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

[xedin]

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?

[Chevel]

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]

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.