Value restriction. The value 'partialTest' has been inferred to have generic type ...

[TIHan]

Code doesn't compile.

Repro steps

1. Put this code at a module level:

open System.Collections

let findCustomer (id:int) = ()

let test (f: int -> 'a) (id: int) (principle: IEnumerable)
    = Some (fun () -> f id)

let partialTest = test findCustomer

Expected behavior

It should compile and give a proper signature.

Actual behavior

It does not compile.

Value restriction. The value 'partialTest' has been inferred to have generic type
    val partialTest : (int -> '_a -> (unit -> unit) option) when '_a :> IEnumerable    
Either make the arguments to 'partialTest' explicit or, if you do not intend for it to be generic, add a type annotation. (using built-in F# compiler)

Known workarounds

Make partialTest generic, let partialTest<'a> = ...

Related information

This compiles at the module level:

let findCustomer (id:int) = ()

let test (f: int -> 'a) (id: int) (principle: string)
    = Some (fun () -> f id)

let partialTest = test findCustomer

[TIHan]

It could be intended design, but I'm sure users would expect this to work since the one using string does. Therefore, I'm deeming this as a bug until said otherwise.

[dsyme]

Yes this is by design - IEnumerable is not sealed where string is, and this causes the value restriction to trigger

[NinoFloris]

@dsyme but there's no flexible type being used here? Why is IEnumerable suddenly being inferred as flexible instead of concretely being IEnumerable?

[TIHan]

I guess my head is exploding, I'm not sure why a sealed type in this case would cause a value restriction. But this would explain when using a record or DU worked.