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Simon Peyton Jones authored
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
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