"too many iterations" causes constraint solving issue.

The following file compiled fine with ghc-7.10, but fails in ghc-8.0.2-rc1 (as well as ghc-8.0.1).

This is a simplified version of a compilation issue with ghc-8 and https://github.com/tensorflow/haskell. It seems similar to #12175 (closed), but even though that was fixed in ghc-8.0.2-rc1, the below code still doesn't compile.

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
module ConstraintTest where

import Data.Int
import Data.Word
import GHC.Exts (Constraint)

import Lens.Family2 ((.~), (&), Lens')
import Lens.Family2.Unchecked (lens)

class MyType a where

instance MyType Int8
instance MyType Int16
instance MyType Int32
instance MyType Int64
instance MyType Word8
instance MyType Word16
instance MyType Word32

-- Require every element in the list to be an instance of 'MyType'.
type family MyTypes (as :: [*]) :: Constraint where
    MyTypes '[] = ()
    MyTypes (a ': as) = (MyType a, MyTypes as)

data Foo = Foo { fooInt :: Int }

class Attr a where
    attr :: Lens' Foo a

instance Attr Int where
    attr = lens fooInt (\f n -> f { fooInt = n })

test :: MyTypes '[Int8,Int16,Int32,Int64,Word8,Word16,Word32]
        => Foo
test = attr .~ (3 :: Int) $ Foo 0

Compilation error:

tensorflow/tests/ConstraintTest.hs:1:1: error:
    solveWanteds: too many iterations (limit = 4)
      Unsolved: WC {wc_simple =
                      [W] hole{a282} :: b_a25A ~ Int (CNonCanonical)}
      New superclasses found
      Set limit with -fconstraint-solver-iterations=n; n=0 for no limit

Some ways to change the test to make the compilation succeed:

• Pass "-fconstraint-solver-iterations=0" to ghc.
• Shorten the type-level list in the constraint of test.
• Replace Lens.Family2 with Lens.Micro from the microlens package. I think this is because Lens.Family2's version of (.~) is higher-order than Lens.Micro: https://hackage.haskell.org/package/lens-family-1.2.1/docs/Lens-Family2.html#v:.-126- https://hackage.haskell.org/package/microlens-0.4.7.0/docs/Lens-Micro.html#v:.-126-

Trac metadata

Trac field	Value
Version	8.0.2-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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