[commit: ghc] ghc-8.6: Instances in no-evidence implications (61adfbe)

[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/61adfbe6f9926daf06031b7da2522f73addf75dc/ghc

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

commit 61adfbe6f9926daf06031b7da2522f73addf75dc
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Fri Jun 22 11:27:47 2018 +0100

    Instances in no-evidence implications
    
    Trac #15290 showed that it's possible that we might attempt to use a
    quantified constraint to solve an equality in a situation where we
    don't have anywhere to put the evidence bindings.  This made GHC crash.
    
    This patch stops the crash, but still rejects the pogram.  See
    Note [Instances in no-evidence implications] in TcInteract.
    
    Finding this bug revealed another lurking bug:
    
    * An infelicity in the treatment of superclasses -- we were expanding
      them locally at the leaves, rather than at their binding site; see
      (3a) in Note [The superclass story].
    
      As a consequence, TcRnTypes.superclassesMightHelp must look inside
      implications.
    
    In more detail:
    
    * Stop the crash, by making TcInteract.chooseInstance test for
      the no-evidence-bindings case.  In that case we simply don't
      use the instance.  This entailed a slight change to the type
      of chooseInstance.
    
    * Make TcSMonad.getPendingScDicts (now renamed getPendingGivenScs)
      return only Givens from the /current level/; and make
      TcRnTypes.superClassesMightHelp look inside implications.
    
    * Refactor the simpl_loop and superclass-expansion stuff in
      TcSimplify.  The logic is much easier to understand now, and
      has less duplication.
    
    (cherry picked from commit 32eb41994f7448caf5fb6b06ed0678d79d029deb)


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

61adfbe6f9926daf06031b7da2522f73addf75dc
 compiler/typecheck/TcCanonical.hs                  |   9 ++
 compiler/typecheck/TcInteract.hs                   |  46 +++++--
 compiler/typecheck/TcRnTypes.hs                    |  28 ++++-
 compiler/typecheck/TcSMonad.hs                     |  46 ++++---
 compiler/typecheck/TcSimplify.hs                   | 134 +++++++++------------
 testsuite/tests/quantified-constraints/T15290.hs   |  35 ++++++
 testsuite/tests/quantified-constraints/T15290a.hs  |  35 ++++++
 .../tests/quantified-constraints/T15290a.stderr    |  22 ++++
 testsuite/tests/quantified-constraints/all.T       |   3 +
 9 files changed, 251 insertions(+), 107 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 61adfbe6f9926daf06031b7da2522f73addf75dc