Allow Type Augmentation Extension on composite types

[Swoorup]

I propose we allow adding intrinsic/optional extensions on type augmentation or composite data types. This makes it a bit more natural to extend composite types analogous to using intrinsic/optional extension on single data type.

Will stress that this is coming purely to make writing type extensions more natural and doesn't care much about interoperability with C#. Type aliases would also simply expand to the underlying type and create extension methods.

type int list with 
    member this.Print() = 
        printfn "%A" this

The existing way of approaching this problem in F# is using Extension methods:

[<Extension>]
type IntExtensions = 
    [<Extension>]
    static member Print(_this: int list) = 
        printfn "%A" _this

Pros and Cons

The advantages of making this adjustment to F# are able

• Consistency with existing F# way to declare intrinsic or optional type extensions.
• Trait like syntax.

The disadvantages of making this adjustment to F# are

• Multiple ways to achieve the same thing
• Brings question on if all extensions essentially compile to C# extension methods?

Extra information

Estimated cost (XS, S, M, L, XL, XXL): M-XL

Related suggestions: (put links to related suggestions here)

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• [x] This is not a question (e.g. like one you might ask on stackoverflow) and I have searched stackoverflow for discussions of this issue
• [x] I have searched both open and closed suggestions on this site and believe this is not a duplicate
• [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:

• [x] This is not a breaking change to the F# language design
• [ ] I or my company would be willing to help implement and/or test this

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