Fable 4 cannot recognize generic property type parameter, when it is not declared on the property explicitly

[ArtemyB]

Description

When a type has a property with implicit generic type parameter, Fable 4 reports an error type parameter is not defined. However F# compiler and Fable 3 handles compilation successfully.

Repro code

Example code:

type Example1 =
    abstract implicitlyGenericProperty1 : 't with get, set
    abstract implicitlyGenericProperty2 : ('t -> string) with get, set
    abstract explicitlyGenericProperty1<'t> : 't with get, set
    abstract explicitlyGenericProperty2<'t> : ('t -> string) with get, set
    abstract implicitlyGenericMethod : 't -> string
    abstract explicitlyGenericMethod<'t> : 't -> string

REPL link

Expected and actual results

In the example above the members implicitlyGenericProperty1 and implicitlyGenericProperty2 provokes the error while the other slightly different members doesn't. The expected behavior is to have the whole code being compiled successfully as it happens with regular F# compiler and Fable 3.

Related information

• Fable version: at least 4.0.0-theta-008, 4.0.0-theta-012, 4.0.0-theta-014
• Operating system: Windows 10 21H1

[kerams]

dotnet/fsharp#12198
fable-compiler/fable-browser#79

This is no longer allowed as it didn't make sense (produced invalid IL) in the first place.

[ArtemyB]

@kerams so that's me who is wrong, not Fable 4 - what a twist 🤔 And just because the issue has been fixed quite recently, I don't see the error in environments other than Fable 4 as I understand. So the problem relates only to inferred property parameters -- there is nothing wrong with explicitly set generic property parameters or inferred (implicit) generic method parameters, am I right?

[kerams]

Properties may not be generic.

I don't see the error in environments other than Fable 4

Fable 4 is using the bleeding edge version of FCS, so it includes the recent fix which prohibits generic properties.

[ArtemyB]

Properties may not be generic.

Then abstract explicitlyGenericProperty1<'t> : 't with get, set in my example should produce the error too, isn't it? However it does not now, and I've even used that fact in my Fable code as a fix of the issue.

[kerams]

Hmm, I don't know. That should not work either in my opinion. It's basically the same thing.

Doesn't work

interface X
{
   public T GetT<T> { get; }
}

Works with a class-level generic type

interface X<T>
{
   public T GetT { get; }
}

@vzarytovskii, thoughts?

[kerams]

Yeah, explicit type parameters are definitely wrong. Just try it with get only. I've submitted a PR forbidding this use case too.

[ArtemyB]

Got it, @kerams, thank you for the clarification 👍 Most likely I wouldn't have discovered the true origin of the issue by myself any time soon. Seems like now it can be closed.