infinite loop on unification with a type family context

[patrick_premont]

The following throws ghc (and ghci) in a loop (ghc-7.0.0.20100925 for i386-unknown-mingw32) and it eventually aborts with "ghc.exe: out of memory".

> {-# LANGUAGE TypeFamilies, Rank2Types, FlexibleContexts #-}

> type family T a

> t2 :: forall a. ((T a ~ a) => a) -> a
> t2 = t
  
> t :: forall a. ((T a ~ a) => a) -> a
> t = undefined

Using ghc 6.12.1 we do not get a loop, but a puzzling error:

typeBug.lhs:9:7:

Couldn't match expected type `(T a ~ a) => a'

against inferred type `(T a ~ a) => a'

Expected type: (T a ~ a) => a Inferred type: (T a ~ a) => a

In the expression: t In the definition of `t2': t2 = t

I presume that the non-injectivity of type family T triggers the problem however the following produces no errors in either version of the compiler.

> ok2 :: forall a. T a -> a
> ok2 = ok
  
> ok :: forall a. T a -> a
> ok = undefined

[patrick_premont]

Code for the bug report

[simonpj]

Thanks for the example. This seems comprehensively fixed by the latest wave of typechecker fixes.

I've added a regression test though!

Simon