Pattern matching on GADT does not refine type family parameters

Compiling the following code

type family R (x :: k) (y :: k) (prf :: x :~: y) :: x :~: y where
  R x y Refl = Refl

fails with the error

Temp.hs:49:9: error:
    • Expected kind ‘x :~: y’, but ‘Refl’ has kind ‘x :~: x’
    • In the third argument of ‘R’, namely ‘Refl’
      In the type family declaration for ‘R’
   |
49 |   R x y Refl = Refl
   |         ^^^^

where Refl is defined as

data a :~: b where
  Refl :: a :~: a

I would expect pattern-matching on Refl to bring the equality x ~ y into scope. It seems like it not only doesn't do that, but the pattern match itself is rejected.

(both 8.2.2 and HEAD)

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information