I propose we allow:

type C(?a) =
    let b = defaultValueArg a 2 // Error here
    member _.B = b

error FS0001: This expression was expected to have type
    ''a voption'    
but here has type
    ''b option'    

The existing way of approaching this problem in F# is using reference options.

Pros and Cons

The advantages of making this adjustment to F# are

1. More structs, less GC pressure
2. Allows for more optimisation
3. In line with recent changes to allow ValueOptions to be used as active pattern return types

The disadvantage of making this adjustment to F# is that existing code ?a will have another potential type for all of us to learn about.

Extra information

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

Related suggestions: all are ValueOption parity features.

• Struct representation for active patterns [ RFC FS-1039 ] #612
• ValueOption parity with options #703
• Add List.chooseV, Seq.tryPickV, etc. for ValueOption #739

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• This is not a question (e.g. like one you might ask on stackoverflow) and I have searched stackoverflow for discussions of this issue
• I have searched both open and closed suggestions on this site and believe this is not a duplicate
• 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:

• 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

For Readers

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