[commit: ghc] master: Only delete instances when merging when there is an exact match. (9df4ce4)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/9df4ce4f89b6d999822ae30ff777bdaf26665c06/ghc

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

commit 9df4ce4f89b6d999822ae30ff777bdaf26665c06
Author: Edward Z. Yang <ezyang at cs.stanford.edu>
Date:   Fri Oct 14 00:11:10 2016 -0700

    Only delete instances when merging when there is an exact match.
    
    Summary:
    Previously, we deleted if the heads matched, which meant that
    we effectively were picking an arbitrary instance if there
    were incompatible instances.  The new behavior makes more sense,
    although without incoherent instances you are unlikely to
    be able to do anything useful with the instances.
    
    Signed-off-by: Edward Z. Yang <ezyang at cs.stanford.edu>
    
    Test Plan: validate
    
    Reviewers: austin, bgamari
    
    Subscribers: thomie
    
    Differential Revision: https://phabricator.haskell.org/D2596


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

9df4ce4f89b6d999822ae30ff777bdaf26665c06
 compiler/backpack/RnModIface.hs  |  5 ++-
 compiler/typecheck/TcBackpack.hs | 88 ++++++++++++++++++++++++++++++++--------
 compiler/types/InstEnv.hs        |  9 ++--
 3 files changed, 78 insertions(+), 24 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 9df4ce4f89b6d999822ae30ff777bdaf26665c06