[GHC] #14396: Hs-boot woes during family instance consistency checks

[GHC]

#14396: Hs-boot woes during family instance consistency checks
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           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider this set of modules (related to #13981 but not the same)
 {{{
 {-# LANGUAGE TypeFamilies #-}
 module Fam where
   type family XListPat a

 {-# LANGUAGE TypeFamilies #-}
 module T1 where
   import Fam
   import {-# SOURCE #-} T( SyntaxExpr )
   type instance XListPat Int = SyntaxExpr

 {-# LANGUAGE TypeFamilies #-}
 module T2 where
   import Fam
   type instance XListPat Bool = Int

 -- T.hs-boot
 module T where
   data SyntaxExpr = S

 -- T.hs
 module T where
   import T1
   import T2
   data SyntaxExpr = S
 }}}
 Compiled with GHC 8.0, 8.2, and HEAD we get
 {{{
 ghc.exe: panic! (the 'impossible' happened)
   (GHC version 8.0.2 for x86_64-unknown-mingw32):
         tcIfaceGlobal (local): not found
   You are in a maze of twisty little passages, all alike.
   While forcing the thunk for TyThing SyntaxExpr
   which was lazily initialized by initIfaceTcRn,
   I tried to tie the knot, but I couldn't find SyntaxExpr
   in the current type environment.
   If you are developing GHC, please read Note [Tying the knot]
   and Note [Type-checking inside the knot].
   Consider rebuilding GHC with profiling for a better stack trace.
   Contents of current type environment: []
 }}}
 Reason:

 * After renaming, but before type checking, we try to do
   family-instance consistency checking in
 `FamInst.checkFamInstConsistency`

 * To do so we have to pull in the axioms from `T1` and `T2`.

 * Then we poke on those axioms, to check consistency, we pull in both
   LHS and RHS of the type instances.

 * Alas that pulls on `SyntaxExpr`, which we have not yet typechecked.

 I don't think it's enough to make lazier the loading of the RHS of the
 axiom, because I think `checkFamInstConsistency` ends up looking at
 the RHS too. See the call to `compatibleBranches` in
 `lookupFamInstEnvConflicts`.

 This setup is actually used in Alan's `wip/ttg-2017-10-13` branch
 for Trees That Grow.  Here module `T` is `HsExpr`, `T1` is `HsPat`.
 And indeed GHC 8.0 crashes when compiling this branch.  SO it's
 becoming a real problem.

 Generally I'm concerned that #13981 may also become more pressing;
 and #14080 is still open

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14396>
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