FS0193: ConstraintSolver error

[CharlesTaylor7]

I'm getting a compiler error message of:

error FS: internal error: Exception of type 'Microsoft.FSharp.Compiler.ConstraintSolver+LocallyAbortOperationThatFailsToResolveOverload' was thrown.

Reproduce steps

Minimal Example in Sharp Lab

Expected behavior

Compiler should report type constraint mismatch for the function test.

Actual behavior

Compiler exception when solving the constraint.

Known workarounds

Deleting either of the * overloads for Polynomial causes the compiler to display a sensible error. Replacing the float literals of 1.0 & 0.5 with decimal literals 1.0m & 0.5m fixes the compiler errors.

[cartermp]

Further reduced repro:

type Complex = unit

type Polynomial () =
    static member (*) (s: decimal, p: Polynomial) : Polynomial = failwith ""
    static member (*) (s: Complex, p: Polynomial) : Polynomial = failwith ""

module Foo =
    let test t (p: Polynomial) = (1.0 - t) * p

Quick look shows that the error doesn't recover, so that's likely the bug.

[cartermp]

Leaking of this internal error is a regression.

F# 4.0: https://repl.it/@cartermp/GlisteningBoilingQuagga

[forki]

Is it solved with @TIHan's latest recovery fix? Phillip Carter <notifications@github.com> schrieb am Di., 4. Sep. 2018, 17:59:

…

Leaking of this internal error is a regression. F# 4.0: ***@***.***/GlisteningBoilingQuagga — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#5580 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AADgNDeGPyky-h9LeOqq2aw6--_PTNZ3ks5uXqNUgaJpZM4WYEIq> .

[TIHan]

@forki it is not resolved with the recovery fix. I just tested it.

[TIHan]

I found the PR that caused this, and the exact changes: https://github.com/Microsoft/visualfsharp/pull/1650/files#diff-641da7c52ffc888699888ebd03559cb5R1285

I came across this by looking at all the history of changes in ConstraintSolver.fs.

Reverting lines 1285 - 1288 will fix this issue. I need to look at this more to see what it is doing.

[TIHan]

Hmm. This seems more complex than I thought. Apparently that PR was reverted, #4173, but then was re-reverted, #4347 due to this: #4343

[TIHan]

Quote from @dsyme regarding #1650

I'm not sure I understand. #1650 has been in for 18 months now and is now the "stable" state. I can see we need to remove the internal error showing above, but we need to do it in a way that doesn't change what does and doesn't compile.

It looks like we were aware of this internal error, but have no found a proper fix to resolve it. Reverting the PR causes other issues, so we have to figure out exactly what is going on to stop the internal error.

[jindraivanek]

Found another repro for this internal error.

FAKE5 script:

#r "paket: nuget Giraffe //"
open System.Threading.Tasks
open FSharp.Control.Tasks.ContextInsensitive

let foo t = task {
    let! t = t
    return t
    }

Error FS0071: Type constraint mismatch when applying the default type 'Task<'a>' for a type inference variable. internal error: Exception of type 'Microsoft.FSharp.Compiler.ConstraintSolver+LocallyAbortOperationThatFailsToResolveOverload' was thrown. Consider adding further type constraints

Annotating t with t: Task<'a> fix error.

[dsyme]

Would be good to have a repro that wasn't a FAKE 5 script i.e. a normal F# project ZIP with exact version of Giraffe

thanks