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Simon Peyton Jones authored
Trac #14164 made GHC loop, a pretty serious error. It turned out that Unify.niFixTCvSubst was looping forever, because we had a substitution like a :-> ....(b :: (c :: d)).... d :-> ... We correctly recognised that d was free in the range of the substitution, but then failed to apply it "deeeply enough" to the range of the substiuttion, so d was /still/ free in the range, and we kept on going. Trac #9106 was caused by a similar problem, but alas my fix to Trac #9106 was inadequate when the offending type variable is more deeply buried. Urk. This time I think I've fixed it! It's much more subtle than I though, and it took most of a long train journey to figure it out. I wrote a long note to explain: Note [Finding the substitution fixpoint]
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