Specialise broken for implicit parameters

The tcrun019 test case is failing with optimization on. Here is an even simpler test case:

{-# LANGUAGE ImplicitParams, RankNTypes #-}

module Main where

f :: (?x :: Int) => ((?x :: Int) => Int) -> Bool -> Int
f g False = g
f g True = let ?x = ?x + 1
           in f g False

h :: (?x :: Int) => Int
h = ?x

main :: IO ()
main = print (let ?x = 0 in f h True)

What happens is that f gets incorrectly specialized, and, as a result, the recursive call to f uses the old binding for the implicit parameter.

I think the problem was introduced with the refactoring in 5a8ac0f8, which broke the test for implicit parameter constraints in Specialise.lhs.

The attached patch seems to fix it.

Trac metadata

Trac field	Value
Version	7.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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