This construct causes code to be less generic than indicated by the type annotations. The type variable 'TRequest' has been constrained to be type ''TRequest'

[nkosi23]

I have the following code defined in an F# class:

 member private this.ExecuteRequest<'TRequest, 'TResponse>
        (endpoint: string, request: 'TRequest')
        : Task<'TResponse> =

        let jsonRequestBody =
            JsonSerializer.Serialize<'TRequest>(request, jsonSerializerOptions)

And it produces the following strange warning:

This construct causes code to be less generic than indicated by the type annotations. The type variable 'TRequest' has been constrained to be type ''TRequest'

I have spent over two hours trying to remove it. Even poked around with various LLMs but they were rapidly out of their depth.

Here is the challenge: if i remove the type hint in JsonSerializer.Serialize<'TRequest> the ExecuteRequest<'T1,'T2>() method is constrained to the type passed by the first caller of this function. Namely, when I have another method calling:

let! myVal = this.ExecuteRequest<RequestType1, ResponseType1>("myendpoint", myRequest)

the second method calling:

let! myVal = this.ExecuteRequest<RequestType2, ResponseType2>("myendpoint", myRequest2)

produces a compiler error saying that myRequest2 was expected to be of type RequestType1.

The compiler errors at the callers sites disappear when i add a type hint to JsonSerializer.Serialize<'TRequest>. However i now get the above warning on the line calling JsonSerializer.Serialize. I've tried moving this line to an independent generic inline function called by that method but it didn't help. Boxing constrains the type to obj which is not what i want and this produces a compiler error anyway (and using boxing just feels wrong in this case, like using a trick when none should be needed). Tried many other variations without success.

I'll be honest, I've been very frustrated to run into this issue, while generic functions, generalizations etc... are supposed to be the strong side of F#, and such code would be trivial in C#. I suspect it has to do with interop between F# generics and C# generics, but still, very frustrating to need to bang my head on something I expected would take me 1 minute. And I still don't know what's going on. I resorted to disabling the warning for the whole file which is not ideal at all but this is the only option i have apparently. I would have preferred to address the root cause if possible.

[majocha]

You have a typo here:

Which would mean 'TRequest' and 'TRequest are two different type parameters 😄

[nkosi23]

Wooooow man... Thank you so much
I've spent half a day on this. And very pleased to see that F# is supporting this case just fine. Thanks a lot!

Edit: in hindsight, I think my IDE automatically matched the ' with a closing ' and I didn't notice. And this is definitely the kind of error i didn't expect, so my eyes didn't catch it despite looking at it dozens of times. I was focused on the generic definition part.