id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
5821	SPECIALISE fails with a cryptic warning	rl		"Example:

{{{
type family T a
type instance T Int = Bool

foo :: Num a => a -> T a
foo = undefined

{-# SPECIALISE foo :: Int -> Bool #-}
}}}

GHC produces this warning:

{{{
    RULE left-hand side too complicated to desugar
      case cobox of _ { GHC.Types.Eq# cobox ->
      (foo @ Int $dNum)
      `cast` (<Int> -> cobox :: (Int -> T Int) ~# (Int -> Bool))
      }
}}}

Given that rewrite rules don't reliably work in the presence of type families, I somewhat suspect that GHC won't be able to generate a sensible specialisation here but it should produce a better diagnostic."	bug	new	normal	7.6.2	Compiler	7.5			illissius@…	Unknown/Multiple	Unknown/Multiple	Incorrect warning at compile-time	Unknown