1. 21 Mar, 2019 1 commit
    • Ryan Scott's avatar
      Reject nested predicates in impredicativity checking · 8d18a873
      Ryan Scott authored
      When GHC attempts to unify a metavariable with a type containing
      foralls, it will be rejected as an occurrence of impredicativity.
      GHC was /not/ extending the same treatment to predicate types, such
      as in the following (erroneous) example from #11514:
      
      ```haskell
      foo :: forall a. (Show a => a -> a) -> ()
      foo = undefined
      ```
      
      This will attempt to instantiate `undefined` at
      `(Show a => a -> a) -> ()`, which is impredicative. This patch
      catches impredicativity arising from predicates in this fashion.
      
      Since GHC is pickier about impredicative instantiations, some test
      cases needed to be updated to be updated so as not to fall afoul of
      the new validity check. (There were a surprising number of
      impredicative uses of `undefined`!) Moreover, the `T14828` test case
      now has slightly less informative types shown with `:print`. This is
      due to a a much deeper issue with the GHCi debugger (see #14828).
      
      Fixes #11514.
      8d18a873
  2. 11 Oct, 2017 1 commit
    • Simon Peyton Jones's avatar
      Remove wc_insol from WantedConstraints · f20cf982
      Simon Peyton Jones authored
      This patch is a pure refactoring, which I've wanted to do for
      some time.  The main payload is
      
      * Remove the wc_insol field from WantedConstraints;
        instead put all the insolubles in wc_simple
      
      * Remove inert_insols from InertCans
        Instead put all the insolubles in inert_irreds
      
      * Add a cc_insol flag to CIrredCan, to record that
        the constraint is definitely insoluble
      
      Reasons
      
      * Quite a bit of code gets slightly simpler
      * Fewer concepts to keep separate
      * Insolubles don't happen at all in production code that is
        just being recompiled, so previously there was a lot of
        moving-about of empty sets
      
      A couple of error messages acutally improved.
      f20cf982
  3. 27 Mar, 2017 1 commit
    • Simon Peyton Jones's avatar
      Fix error-message suppress on given equalities · e0ad55f8
      Simon Peyton Jones authored
      I'd got the logic slightly wrong when reporting type errors
      for insoluble 'given' equalities.  We suppress insoluble givens
      under some circumstances (see Note [Given errors]), but we then
      suppressed subsequent 'wanted' errors because the (suppressed)
      'given' error "won".  Result: no errors at all :-(.
      
      This patch fixes it and
       - Renames TcType.isTyVarUnderDatatype to the more
         perspicuous TcType.isInsolubleOccursCheck
      
      In doing this I realise that I don't understand why we need
      to keep the insolubles partitioned out separately at all...
      but that is for another day.
      e0ad55f8
  4. 12 Oct, 2016 1 commit