[GHC] #15290: QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"

[GHC]

#15290: QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  quantified-
                                     |  constraints/T15290, T15290a
      Blocked By:                    |             Blocking:  9123, 14883
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Simon made a commit (9fc40c733ba8822a04bd92883801b214dee099ca) addressing
 the issue in comment:24, but he accidentally tagged the wrong ticket
 number. Here is the commit:

 {{{
 From 9fc40c733ba8822a04bd92883801b214dee099ca Mon Sep 17 00:00:00 2001
 From: Simon Peyton Jones <simonpj at microsoft.com>
 Date: Mon, 25 Jun 2018 13:20:59 +0100
 Subject: [PATCH] Refactor the kind-checking of tyvar binders

 The refactoring here is driven by the ghastly mess described in
 comment:24 of Trac #1520.  The overall goal is to simplify the
 kind-checking of typev-variable binders, and in particular to narrow
 the use of the "in-scope tyvar binder" stuff,
 which is needed only for associated types: see the new
 Note [Kind-checking tyvar binders for associated types] in TcHsType.

 Now

 * The "in-scope tyvar binder" stuff is done only in
      - kcLHsQTyVars, which is used for the LHsQTyVars of a
        data/newtype, or type family declaration.

      - tcFamTyPats, which is used for associated family instances;
        it now calls tcImplicitQTKBndrs, which in turn usese
        newFlexiKindedQTyVar

 * tcExpicitTKBndrs (which is used only for function signatures,
   data con signatures, pattern synonym signatures, and expression
   type signatures) now does not go via the "in-scope tyvar binder"
   stuff at all.

 While I'm still not happy with all this code, the code is generally
 simpler, and I think this is a useful step forward. It does cure
 the problem too.

 (It's hard to trigger the problem in vanilla Haskell code, because
 the renamer would normally use different names for nested binders,
 so I can't offer a test.)
 ---
  compiler/hsSyn/HsDecls.hs          |  43 +++++--
  compiler/typecheck/TcHsType.hs     | 258
 +++++++++++++++++++++----------------
  compiler/typecheck/TcTyClsDecls.hs |   3 +-
  3 files changed, 178 insertions(+), 126 deletions(-)
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15290#comment:25>
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