Original bug ID: 5736
Reporter: omion
Assigned to: @garrigue
Status: confirmed (set by @garrigue on 2012-08-22T23:27:03Z)
Resolution: open
Priority: normal
Severity: feature
Version: 4.00.0
Target version: later
Category: typing
Related to: #7319
Child of: #5998
Monitored by: @hcarty

Bug description

Currently, if you have a GADT with multiple branches sharing the same type, eg:

type _ gadt =
| Float : float -> float gadt
| Float2 : float -> float gadt
| Int : int -> int gadt

then try to use Float and Float2 in one branch of a function:

let type_of_gadt : type t. t gadt -> string = function
| Float _ | Float2 _ -> "float"
| Int _ -> "int"

this will not pass the type checker, even though Float and Float2 have the same type:

Error: This pattern matches values of type float gadt
       but a pattern was expected which matches values of type t gadt

You would have to have each variant:

let type_of_gadt : type t. t gadt -> string = function
| Float _ -> "float"
| Float2 _ -> "float"
| Int _ -> "int"

or, matching with the default "_" will work since it's not an or-pattern, but this only works well if there is only one type in the GADT that has the problem:

let type_of_gadt : type t. t gadt -> string = function
| Int _ -> "int"
| _ -> "float"

It would be handy to be able to combine the branches for GADT constructors that refer to the same types.