Odd interaction between record update and type families

Consider the following example

{-# LANGUAGE TypeFamilies #-}

type family F a
type family G a

data T1
type instance F T1 = Char
type instance G T1 = Int

data T2
type instance F T2 = Bool
type instance G T2 = Int

data R a = R { x :: F a, y :: G a }

r1 :: R T1
r1 = R { x = 'a', y = 2 }

r2 :: R T2
r2 = r1 { x = True }  -- error: Cannot match T1 with T2

r3 :: R T2
r3 = r1 { x = True, y = y r1 } -- OK

The error for r2 is odd because types R T1 and R T2 differ only in the type of x, so if we start with a value of type R T1, and update the field where the types differ, we should end up with a valid value of type R T2.

The fact that r2 is OK is demonstrated by r3 where we update the other field with its own value, and now the type checker is happy.

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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