Confusing and ambiguous error (FS0368) when implementing an interface with overloads

[abelbraaksma]

If you implement only one member of an interface with overloads without using specific typing you may well receive a very dubious error claiming your override has the wrong number of type parameters:

error FS0368: The member 'Bar<'a0> : 'a0 -> int' does not have the correct number of method type parameters. The required signature is 'Bar : double -> int'.

Repro steps

To see this in action, copy the following snippet into any F# project:

type IOverload =
    abstract member Bar : int -> int
    abstract member Bar : double -> int

type Overload =
    interface IOverload with
        override __.Bar _ = 1    // FS0368

Expected behavior

An error that explains what's going on. There are no type parameters here. Suggested text for error:

Ambiguous match: the generic method Bar matches multiple overloads of the same method, candidates are 'Bar : double -> int', 'Bar : int -> int'. Use a more specific type annotation for overriding the proper method.

Actual behavior

Actual behavior is that you see the following in the editor:

If you compile the project, you get a little more information, but it is still not covering the actual error:

error FS0368: The member 'Bar<'a0> : 'a0 -> int' does not have the correct number of method type parameters. The required signature is 'Bar : double -> int'.

error FS0368: The member 'Bar<'a0> : 'a0 -> int' does not have the correct number of method type parameters. The required signature is 'Bar : int -> int'.

error FS0017: The member 'Bar<'a0> : 'a0 -> int' does not have the correct type to override any given virtual method

Known workarounds

None (unless of course: learning what this error may mean and act on it).

Related information

Seen with:

• Windows 7
• .NET Runtime 4
• VS 2025
• F# 4.0

[forki]

would that help?

[abelbraaksma]

@forki, That looks excellent, yes, that would certainly help! Is this the existing internal build or did you just fix /change this?

Awe, so fast :))))

[forki]

WIP PR at #1907