Pattern `DUCase` matches `DUCase _`

[charlesroddie]

type DU = | DUCase of int
let y =
    function
    | DUCase -> () // This should fail with FS0019: This constructor is applied to 0 argument(s) but expects 1

This type-checks in VS tooling and works in F# interactive. Behaviour is as if the user had written the correct | DUCase _ -> ()

This doesn't apply to active patterns, which fail without arguments as expected.

Tested on VS 2019 16.6.0 and "F# Interactive version 10.9.1.0 for F# 4.7".

SharpLab doesn't have the problem and gives expected error.

[abelbraaksma]

This must be a recent regression. When I try it in FSI in VS 2019 16.5.4 (yep, still had a copy that wasn't updated), I can see:

> type DU = | DUCase of int
let y =
    function
    | DUCase -> ();;

      | DUCase -> ();;
  ------^^^^^^

C:\[...]\stdin(6,7): error FS0019: This constructor is applied to 0 argument(s) but expects 1

And the same code in 16.7.0 Preview 1.0, it succeeds without error. Which suggests somewhere between 16.5.4 and 16.6.0 it was introduced, and not yet fixed in new previews.

EDIT: just updated to Preview 2.0, same there, the error is not shown, but should be.

[KevinRansom]

This is fixed by this recent PR: #9426

[cartermp]

Closing as duplicate of #9420