Hs-boot woes during family instance consistency checks

Consider this set of modules (related to #13981 (closed) 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 (closed) may also become more pressing; and #14080 (closed) is still open

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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