#14363: :type hangs on coerce
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Reporter: Iceland_jack | Owner: (none)
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 8.2.1
Resolution: | Keywords: Roles
Operating System: Unknown/Multiple | Architecture:
| Unknown/Multiple
Type of failure: None/Unknown | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s):
Wiki Page: |
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Comment (by Simon Peyton Jones ):
In [changeset:"3acd6164fea6d4d5d87521a291455a18c9c9a8ee/ghc" 3acd616/ghc]:
{{{
#!CommitTicketReference repository="ghc"
revision="3acd6164fea6d4d5d87521a291455a18c9c9a8ee"
Improve kick-out in the constraint solver
This patch was provoked by Trac #14363. Turned out that we were
kicking out too many constraints in TcSMonad.kickOutRewritable, and
that mean that the work-list never became empty: infinite loop!
That in turn made me look harder at the Main Theorem in
Note [Extending the inert equalities].
Main changes
* Replace TcType.isTyVarExposed by TcType.isTyVarHead. The
over-agressive isTyVarExposed is what caused Trac #14363.
See Note [K3: completeness of solving] in TcSMonad.
* TcType.Make anyRewriteableTyVar role-aware. In particular,
a ~R ty
cannot rewrite
b ~R f a
See Note [anyRewriteableTyVar must be role-aware]. That means
it has to be given a role argument, which forces a little
refactoring.
I think this change is fixing a bug that hasn't yet been reported.
The actual reported bug is handled by the previous bullet. But
this change is definitely the Right Thing
The main changes are in TcSMonad.kick_out_rewritable, and in TcType
(isTyVarExposed ---> isTyVarHead).
I did a little unforced refactoring:
* Use the cc_eq_rel field of a CTyEqCan when it is available, rather
than recomputing it.
* Define eqCanRewrite :: EqRel -> EqRel -> EqRel, and use it, instead
of duplicating its logic
}}}
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14363#comment:6
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