Allow eliding qualification when pattern matching against DUs where type is known

[cmeeren]

I propose we allow eliding module/type qualifications when pattern matching against DUs where the type is known.

Today, for records, F# is smart enough to not require qualifying the type when it knows which type you are matching against:

type A = { X: int }

type B = { X: int }

let f () = { A.X = 1 }

match f () with
| { X = x } -> x

However, this is not the case for DUs. From a dev perspective, I see no reason why I shouldn't be able to do this:

type A = | Case1

type B = | Case1

let f () = A.Case1

match f () with
| Case1 -> ()

However, this fails on Case1 on the last line because it thinks I am pattern matching against B:

So currently, we must qualify the case:

match f () with
| A.Case1 -> ()

With longer type names nested inside modules, like SelfAttestedCourseCompletion.CreateError, the pattern matching starts to get fairly verbose.

Interop with active patterns

If there is an active pattern in scope with the same name, that should of course still take precedence.

Pros and Cons

The advantages of making this adjustment to F# are: Consistency with record features, less verbosity without loss of clarity.

The disadvantages of making this adjustment to F# are: None?

Extra information

Estimated cost (XS, S, M, L, XL, XXL): Don't know, maybe S-M?

Related suggestions: (put links to related suggestions here)

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• This is not a breaking change to the F# language design
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