[commit: ghc] ghc-8.2: Fix bug in the short-cut solver (0461305)

[git at git.haskell.org]

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

On branch  : ghc-8.2
Link       : http://ghc.haskell.org/trac/ghc/changeset/046130558d825dd05734238c8c87bae4a148bdc1/ghc

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

commit 046130558d825dd05734238c8c87bae4a148bdc1
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Mon Oct 2 15:58:46 2017 +0100

    Fix bug in the short-cut solver
    
    Trac #13943 showed that the relatively-new short-cut solver
    for class constraints (aka -fsolve-constant-dicts) was wrong.
    In particular, see "Type families" under Note [Shortcut solving]
    in TcInteract.
    
    The short-cut solver recursively solves sub-goals, but it doesn't
    flatten type-family applications, and as a result it erroneously
    thought that C (F a) cannot possibly match (C 0), which is
    simply untrue.  That led to an inifinte loop in the short-cut
    solver.
    
    The significant change is the one line
    
    +                 , all isTyFamFree preds  -- See "Type families" in
    +                                          -- Note [Shortcut solving]
    
    but, as ever, I do some other refactoring.  (E.g. I changed the
    name of the function to shortCutSolver rather than the more
    generic trySolveFromInstance.)
    
    I also made the short-cut solver respect the solver-depth limit,
    so that if this happens again it won't just produce an infinite
    loop.
    
    A bit of other refactoring, notably moving isTyFamFree
    from TcValidity to TcType
    
    (cherry picked from commit a8fde1831f4b99885b8ed444f9cd7dffd9252150)


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046130558d825dd05734238c8c87bae4a148bdc1
 compiler/typecheck/TcInteract.hs                   | 139 +++++++++++++--------
 compiler/typecheck/TcType.hs                       |   8 +-
 compiler/typecheck/TcValidity.hs                   |   4 -
 testsuite/tests/typecheck/should_compile/T13943.hs |  68 ++++++++++
 testsuite/tests/typecheck/should_compile/all.T     |   1 +
 5 files changed, 161 insertions(+), 59 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 046130558d825dd05734238c8c87bae4a148bdc1