Fix a TyVar bug in the flattener
A year ago I gave up on trying to rigorously separate TyVars from TcTyVars, and instead allowed TyVars to appear rather more freely in types examined by the constraint solver: commit 18d0bdd3 Author: Simon Peyton Jones <email@example.com> Date: Wed Nov 23 16:00:00 2016 +0000 Allow TyVars in TcTypes See Note [TcTyVars in the typechecker] in TcType. However, TcFlatten.flatten_tyvar1 turned out to treat a TyVar specially, and implicitly assumed that it could not have an equality constraint in the inert set. Wrong! This caused Trac #14450. Fortunately it is easily fixed, by deleting code.
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