• Simon Peyton Jones's avatar
    Fix two cloning-related bugs · 016a0bd1
    Simon Peyton Jones authored
    Crikey!  Not just one but two bugs in type variable cloning,
    both dating from the days before PolyKinds.  Both were shown up
    by Trac #11330.
    1. In SetLevels, when floating a case expression we must clone its
       binders, *and* do so in a telescope-aware way, because the
       constructor may bind a kind variable that appears in the kind
       of a type variable.
       Instead of doing this (wrongly) by steam, call CoreSubst.cloneBndrs.
       I added Notes and did other refactoring at the same time.
    2. It turned out that CoreSubst.cloneBndrs calls TyCoRep.cloneTyVarBndr,
       and that too was bogus!  It didn't substitute in the kind of the
       TyVar being cloned.  There was even a comment to say "variables can't
       appear in kinds".  Thta hasn't been true for a long time now.
    Easily fixed.
    Interestingly, I then found that test
    was emitting a new inexhaustive-pattern-match warning.  Sure enough
    it was valid!  So the lack of cloning in cloneTyVarBndr really was
    causing an observable bug; just one that we had not observed.
TyCoRep.hs 106 KB