[commit: ghc] ghc-8.4: Fix two pernicious bugs in DeriveAnyClass (d8dbe29)

[git at git.haskell.org]

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

On branch  : ghc-8.4
Link       : http://ghc.haskell.org/trac/ghc/changeset/d8dbe2936c923471a13e214113b0e43222e95592/ghc

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

commit d8dbe2936c923471a13e214113b0e43222e95592
Author: Ryan Scott <ryan.gl.scott at gmail.com>
Date:   Sun Mar 25 14:00:39 2018 -0400

    Fix two pernicious bugs in DeriveAnyClass
    
    The way GHC was handling `DeriveAnyClass` was subtly wrong
    in two notable ways:
    
    * In `inferConstraintsDAC`, we were //always// bumping the `TcLevel`
      of newly created unification variables, under the assumption that
      we would always place those unification variables inside an
      implication constraint. But #14932 showed precisely the scenario
      where we had `DeriveAnyClass` //without// any of the generated
      constraints being used inside an implication, which made GHC
      incorrectly believe the unification variables were untouchable.
    * Even worse, we were using the generated unification variables from
      `inferConstraintsDAC` in every single iteration of `simplifyDeriv`.
      In #14933, however, we have a scenario where we fill in a
      unification variable with a skolem in one iteration, discard it,
      proceed on to another iteration, use the same unification variable
      (still filled in with the old skolem), and try to unify it with
      a //new// skolem! This results in an utter disaster.
    
    The root of both these problems is `inferConstraintsDAC`. This patch
    fixes the issue by no longer generating unification variables
    directly in `inferConstraintsDAC`. Instead, we store the original
    variables from a generic default type signature in `to_metas`, a new
    field of `ThetaOrigin`, and in each iteration of `simplifyDeriv`, we
    generate fresh meta tyvars (avoiding the second issue). Moreover,
    this allows us to more carefully fine-tune the `TcLevel` under which
    we create these meta tyvars, fixing the first issue.
    
    Test Plan: make test TEST="T14932 T14933"
    
    Reviewers: simonpj, bgamari
    
    Reviewed By: simonpj
    
    Subscribers: rwbarton, thomie, carter
    
    GHC Trac Issues: #14932, #14933
    
    Differential Revision: https://phabricator.haskell.org/D4507
    
    (cherry picked from commit 9893042604cda5260cb0f7b674ed5c34b419e403)


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d8dbe2936c923471a13e214113b0e43222e95592
 compiler/typecheck/TcDerivInfer.hs                | 103 ++++++++++++----------
 compiler/typecheck/TcDerivUtils.hs                |  97 +++++++++++++-------
 testsuite/tests/deriving/should_compile/T14932.hs |  23 +++++
 testsuite/tests/deriving/should_compile/T14933.hs |  22 +++++
 testsuite/tests/deriving/should_compile/all.T     |   2 +
 5 files changed, 170 insertions(+), 77 deletions(-)

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    git diff-tree --root --patch-with-stat --no-color --find-copies-harder --ignore-space-at-eol --cc d8dbe2936c923471a13e214113b0e43222e95592