[GHC] #12644: Lack of instantiation in GHC 8.0.1

[GHC]

#12644: Lack of instantiation in GHC 8.0.1
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  typecheck/should_compile/T12644
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by simonpj):

 * status:  new => closed
 * testcase:   => typecheck/should_compile/T12644
 * resolution:   => fixed


Comment:

 Fixed as part of
 {{{
 commit b612da667fe8fa5277fc78e972a86d4b35f98364
 Author: Simon Peyton Jones <simonpj at microsoft.com>
 Date:   Sun Sep 25 15:50:18 2016 +0100

     Fix impredicativity (again)

     This patch fixes Trac #12616.

     Dignosis.  In TcUnify.tc_sub_type_ds we were going to some trouble to
     support co- and contra-variance even for impredicative types.  With
     -XImpredicativeTYpes, this allowed a unification variable to be
     unified with a polytype (probably wrongly) and that caused later
     trouble in the constraint solver, where -XImpredicativeTypes was /not/
     on.  In effect, -XImpredicativeTypes can't be switched on locally.

     Why did we want ImpredicativeTypes locally?  Because the program
     generated by GND for a higher-rank method involved impredicative
     instantation of 'coerce':
           op = coerce op   -- where op has a higher rank type
     See Note [Newtype-deriving instances] in TcGenDeriv.

     Cure.

     1.  It is ghastly to rely on ImpredicativeTypes (a 100% flaky
         feature) to instantiate coerce polymorphically.  Happily we
         now have Visible Type Application, so I've used that instead
         which should be solid and reliable.

     2.  I deleted the code in tc_sub_type_ds that allows the constraint
         solver to "look through" a unification variable to find a
         polytype.  That used to be essential in the days of ReturnTv,
         but it's utterly unreliable and should be consigned to the dustbin
         of history.  (We have ExpType now for the essential uses.)

     Tests involving ImpredicativeTypes are affected, but I'm not worried
     about them... it's advertised as a feature you can't rely on, and
     I want to reform it outright.

  compiler/hsSyn/HsUtils.hs                              |  8 ++++----
  compiler/typecheck/TcDeriv.hs                          |  2 ++
  compiler/typecheck/TcGenDeriv.hs                       | 61
 +++++++++++++++++++++++++++++++------------------------------
  compiler/typecheck/TcUnify.hs                          | 30
 ++++++++----------------------
  testsuite/tests/boxy/Base1.hs                          | 35
 +++++++++++++++++++----------------
  testsuite/tests/boxy/T2193.hs                          |  2 ++
  testsuite/tests/boxy/all.T                             |  4 ++--
  testsuite/tests/deriving/should_compile/T12616.hs      | 21
 +++++++++++++++++++++
  testsuite/tests/deriving/should_compile/all.T          |  1 +
  testsuite/tests/deriving/should_fail/T4846.stderr      |  4 ++--
  testsuite/tests/typecheck/should_compile/T11319.hs     |  3 +++
  testsuite/tests/typecheck/should_compile/T12644.hs     | 14
 ++++++++++++++
  testsuite/tests/typecheck/should_compile/all.T         |  3 ++-
  testsuite/tests/typecheck/should_compile/tc211.hs      |  3 +++
  testsuite/tests/typecheck/should_compile/tc211.stderr  | 78
 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
  testsuite/tests/typecheck/should_fail/T10619.stderr    | 18
 ++++--------------
  testsuite/tests/typecheck/should_fail/T2846b.stderr    |  3 ++-
  testsuite/tests/typecheck/should_fail/T8428.stderr     |  5 ++++-
  testsuite/tests/typecheck/should_fail/all.T            |  2 +-
  testsuite/tests/typecheck/should_fail/tcfail016.stderr |  2 +-
  testsuite/tests/typecheck/should_fail/tcfail165.hs     |  4 +++-
  testsuite/tests/typecheck/should_fail/tcfail165.stderr | 12 ++++++++++++
  testsuite/tests/typecheck/should_fail/tcfail174.hs     |  2 ++
  testsuite/tests/typecheck/should_fail/tcfail174.stderr | 19
 +++++++++++++------
  24 files changed, 230 insertions(+), 106 deletions(-)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12644#comment:2>
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