This will be supported in 5.3 (if there are no complications). The reason why the feature has only been introduced in 5.3 is that the type variables 'a and 'b are not existentially quantified in
type _ foo = Foo : 'a * 'b -> ('a * 'b) foo
since they appear in the type of the variant. The recent PR is 5.3 acknowledge that it is nice to name all type variables introduced by matching other a GADT constructor and extend the support for
function (Foo (type a b) (x, y : a * b)) -> (x, y)
to all type variables introduced by the GADT equation.