[commit: ghc] ghc-8.6: Add nakedSubstTy and use it in TcHsType.tcInferApps (cfc4afa)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : ghc-8.6
Link       : http://ghc.haskell.org/trac/ghc/changeset/cfc4afad16d0eb99d5576cd998bcf473a1bc2af5/ghc

>---------------------------------------------------------------

commit cfc4afad16d0eb99d5576cd998bcf473a1bc2af5
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Mon Jul 9 17:29:22 2018 +0100

    Add nakedSubstTy and use it in TcHsType.tcInferApps
    
    This was a tricky one.
    
    During type checking we maintain TcType:
       Note [The well-kinded type invariant]
    That is, types are well-kinded /without/ zonking.
    
    But in tcInferApps we were destroying that invariant by calling
    substTy, which in turn uses smart constructors, which eliminate
    apparently-redundant Refl casts.
    
    This is horribly hard to debug beause they really are Refls and
    so it "ought" to be OK to discard them. But it isn't, as the
    above Note describes in some detail.
    
    Maybe we should review the invariant?  But for now I just followed
    it, tricky thought it is.
    
    This popped up because (for some reason) when I fixed Trac #15343,
    that exposed this bug by making test polykinds/T14174a fail (in
    Trac #14174 which indeed has the same origin).
    
    So this patch fixes a long standing and very subtle bug.
    
    One interesting point: I defined nakedSubstTy in a few lines by
    using the generic mapType stuff.  I note that the "normal"
    TyCoRep.substTy does /not/ use mapType.  But perhaps it should:
    substTy has lots of $! strict applications in it, and they could
    all be eliminated just by useing the StrictIdentity monad.  And
    that'd make it much easier to experiment with switching between
    strict and lazy versions.
    
    (cherry picked from commit 5067b205a8abb5a9f98335d3a929f647c88c0aa2)


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cfc4afad16d0eb99d5576cd998bcf473a1bc2af5
 compiler/typecheck/TcHsType.hs |  20 +++++---
 compiler/typecheck/TcType.hs   | 108 ++++++++++++++++++++++++++++++-----------
 2 files changed, 94 insertions(+), 34 deletions(-)

Diff suppressed because of size. To see it, use:

    git diff-tree --root --patch-with-stat --no-color --find-copies-harder --ignore-space-at-eol --cc cfc4afad16d0eb99d5576cd998bcf473a1bc2af5