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Simon Peyton Jones authored
After a struggle, I fixed Trac #5113 (again) on the 7.10 branch, by adding an ambient substitution to Subst; see CoreSubst, esp Note [IdSubstEnv]. This allowed me to do the impedence-matching in SimplEnv.substExpr efficiently (fixing #10370) as well correctly (fixing the latest problem with #5113). This cost me more time than I like to say. Sigh.
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