• Simon Peyton Jones's avatar
    Fix bogus type of case expression · 0dad81ca
    Simon Peyton Jones authored
    Issue #17056 revealed that we were sometimes building a case
    expression whose type field (in the Case constructor) was bogus.
    
    Consider a phantom type synonym
       type S a = Int
    and we want to form the case expression
       case x of K (a::*) -> (e :: S a)
    We must not make the type field of the Case constructor be (S a)
    because 'a' isn't in scope.  We must instead expand the synonym.
    
    Changes in this patch:
    
    * Expand synonyms in the new function CoreUtils.mkSingleAltCase.
    
    * Use mkSingleAltCase in MkCore.wrapFloat, which was the proximate
      source of the bug (when called by exprIsConApp_maybe)
    
    * Use mkSingleAltCase elsewhere
    
    * Documentation
        CoreSyn   new invariant (6) in Note [Case expression invariants]
        CoreSyn   Note [Why does Case have a 'Type' field?]
        CoreUtils Note [Care with the type of a case expression]
    
    * I improved Core Lint's error reporting, which was pretty
      confusing in this case, because it didn't mention that the offending
      type was the return type of a case expression.
    
    * A little bit of cosmetic refactoring in CoreUtils
    0dad81ca
Code owners : Ben Gamari
CorePrep.hs 66.6 KB