• Richard Eisenberg's avatar
    Track type variable scope more carefully. · faec8d35
    Richard Eisenberg authored
    The main job of this commit is to track more accurately the scope
    of tyvars introduced by user-written foralls. For example, it would
    be to have something like this:
      forall a. Int -> (forall k (b :: k). Proxy '[a, b]) -> Bool
    In that type, a's kind must be k, but k isn't in scope. We had a
    terrible way of doing this before (not worth repeating or describing
    here, but see the old tcImplicitTKBndrs and friends), but now
    we have a principled approach: make an Implication when kind-checking
    a forall. Doing so then hooks into the existing machinery for
    preventing skolem-escape, performing floating, etc. This also means
    that we bump the TcLevel whenever going into a forall.
    The new behavior is done in TcHsType.scopeTyVars, but see also
    TcHsType.tc{Im,Ex}plicitTKBndrs, which have undergone significant
    rewriting. There are several Notes near there to guide you. Of
    particular interest there is that Implication constraints can now
    have skolems that are out of order; this situation is reported in
    A major consequence of this is a slightly tweaked process for type-
    checking type declarations. The new Note [Use SigTvs in kind-checking
    pass] in TcTyClsDecls lays it out.
    The error message for dependent/should_fail/TypeSkolEscape has become
    noticeably worse. However, this is because the code in TcErrors goes to
    some length to preserve pre-8.0 error messages for kind errors. It's time
    to rip off that plaster and get rid of much of the kind-error-specific
    error messages. I tried this, and doing so led to a lovely error message
    for TypeSkolEscape. So: I'm accepting the error message quality regression
    for now, but will open up a new ticket to fix it, along with a larger
    error-message improvement I've been pondering. This applies also to
    dependent/should_fail/{BadTelescope2,T14066,T14066e}, polykinds/T11142.
    Other minor changes:
     - isUnliftedTypeKind didn't look for tuples and sums. It does now.
     - check_type used check_arg_type on both sides of an AppTy. But the left
       side of an AppTy isn't an arg, and this was causing a bad error message.
       I've changed it to use check_type on the left-hand side.
     - Some refactoring around when we print (TYPE blah) in error messages.
       The changes decrease the times when we do so, to good effect.
       Of course, this is still all controlled by
    Fixes #14066 #14749
    Test cases: dependent/should_compile/{T14066a,T14749},
TcErrors.hs 144 KB