"Duplicate method ... same signature" error occurs for member names other than op_Implicit

Consider this code:

type A =
  | A of int
  static member M(A x) : int = x
  static member M(A x) : string = x.ToString()

This leads to the following compilation error:

Duplicate method. The method ‘M’ has the same name and signature as another method in type ‘A’.

I assume “signature” refers to the parameters and ignores the return type. However, using the name op_Implicit works fine:

type A =
  | A of int
  static member op_Implicit(A x) : int = x
  static member op_Implicit(A x) : string = x.ToString()

Why should it depend on the name? Is this a bug?

My use-case is that I am using some SRTP helpers to convert between compatible types, but I’d like to move away from relying on op_Implicit for these helpers in order to force myself to use these helpers. With op_Implicit, I get automatic conversion at method call sites (but I don’t want to enable the warning/error for this, since I want that implicit conversion for some built-in .NET APIs/types).

Related information

• Windows 11
• Rider 2022.2.3

[dsyme]

This is by design - overloading by return type is only allowed for op_Implicit and op_Explicit, it’s simply not allowed for other members.

Return types are only taken into account on SRTP constraint solving.

op_Implicit/Explicit are just so different to other members as they nearly only ever invoked through this constraint solving or other F# 6 implicit conversion resolution. Plus you simply can’t define useful sets of these overloads without overloading by return type.

For other methods, we ban overloading by return type because it would give the false impression that return types are taken into account bin overload resolution.

I agree there’s a corner case migration problem here making it hard to migrate away from op_Implicit. I guess some attribute to allow overloading by return type could make sense. I have a feeling that’s been proposed before, need to search for it.

Anyway, currently by design and would need to go via a language suggestion

[NinoFloris]

I have a feeling that’s been proposed before, need to search for it.

You’re correct, it lives in the trait extension PR https://github.com/dotnet/fsharp/pull/6805 as AllowOverloadOnReturnTypeAttribute. Which would close the proposal https://github.com/fsharp/fslang-suggestions/issues/820