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Simon Peyton Jones authored
tcSimplifyRuleLhs is a cut-down constraint simplifier, intended for use in RULE left-hand-sides. But it was written before implication constraints, and the exmaple of this bug report shows that when higher rank types are involved we need to be a bit cleverer. The whole business of simplifying constraints on rule LHSs is a bit of a hack; but for a good reason. See the comments with tcSimplifyRuleLhs. This patch at least cures the crash.
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