occurs check should not apply to type families

This code:

{-# LANGUAGE TypeFamilies #-}
type family Id a
cid :: a ~ Id a => a -> a
cid x = x
cundefined = cid undefined

gives this error:

test.hs:5:14:
    Occurs check: cannot construct the infinite type: a0 = Id a0
    In the expression: cid undefined
    In an equation for `cundefined': cundefined = cid undefined

whereas I think it should be possible to infer the type cundefined :: a ~ Id a => a. Equations with a variable on one side and a type family at the head of the other side should become constraints rather than being subject to the occurs check.

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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