F# - Extending .NET BCL nested classes

Question

The class System.Collections.Immutable.ImmutableHashSet<'t> has a nested type Builder. We can use it in F# without issues.

What I would like to do is add an extension method to that class in F#, like

type ImmutableHashSet<'t>.Builder with
    member this.NewMember() = ...

but this is not valid F# syntax.

If I extract the fsi I see the following definition

namespace System.Collections.Immutable.ImmutableHashSet`1
type Builder<'T> = ...

however the syntax

type ``System.Collections.Immutable.ImmutableHashSet`1``.Builder with
    member this.NewMember() = ...

is also non valid.

Is there a way to write this extension in F#?

Answer 1

Back-ticks are confusing the markdown, so to clarify what @JL0PD wrote, the following seems to work:

open System.Collections.Immutable

type ``ImmutableHashSet`1``.Builder<'a> with
    member _.NewMember() = printfn "Hello world"

let builder = ImmutableHashSet.CreateBuilder<int>()
builder.NewMember()   // Hello world

Exposing the mangled name like this is surprising to me, so it's possible that this only works by accident and could change in the future.