Another major constraint-solver refactoring
This patch takes further my refactoring of the constraint solver, which I've been doing over the last couple of months in consultation with Richard. It fixes a number of tricky bugs that made the constraint solver actually go into a loop, including Trac #12526 Trac #12444 Trac #12538 The main changes are these * Flatten unification variables (fmvs/fuvs) appear on the LHS of a tvar/tyvar equality; thus fmv ~ alpha and not alpha ~ fmv See Note [Put flatten unification variables on the left] in TcUnify. This is implemented by TcUnify.swapOverTyVars. * Don't reduce a "loopy" CFunEqCan where the fsk appears on the LHS: F t1 .. tn ~ fsk where 'fsk' is free in t1..tn. See Note [FunEq occurs-check principle] in TcInteract This neatly stops some infinite loops that people reported; and it allows us to delete some crufty code in reduce_top_fun_eq. And it appears to be no loss whatsoever. As well as fixing loops, ContextStack2 and T5837 both terminate when they didn't before. * Previously we generated "derived shadow" constraints from Wanteds, but we could (and sometimes did; Trac #xxxx) repeatedly generate a derived shadow from the same Wanted. A big change in this patch is to have two kinds of Wanteds: [WD] behaves like a pair of a Wanted and a Derived [W] behaves like a Wanted only See CtFlavour and ShadowInfo in TcRnTypes, and the ctev_nosh field of a Wanted. This turned out to be a lot simpler. A [WD] gets split into a [W] and a [D] in TcSMonad.maybeEmitShaodow. See TcSMonad Note [The improvement story and derived shadows] * Rather than have a separate inert_model in the InertCans, I've put the derived equalities back into inert_eqs. We weren't gaining anything from a separate field. * Previously we had a mode for the constraint solver in which it would more aggressively solve Derived constraints; it was used for simplifying the context of a 'deriving' clause, or a 'default' delcaration, for example. But the complexity wasn't worth it; now I just make proper Wanted constraints. See TcMType.cloneWC * Don't generate injectivity improvement for Givens; see Note [No FunEq improvement for Givens] in TcInteract * solveSimpleWanteds leaves the insolubles in-place rather than returning them. Simpler. I also did lots of work on comments, including fixing Trac #12821.
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