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See new Note [Use only the best local instance] in
GHC.Tc.Solver.Interact.
This commit also refactors the InstSC/OtherSC mechanism
slightly.
Close #20582.
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Previously, we reported things wrong with
f :: (Eq a, Ord a) => a -> Bool
f x = x == x
saying that Eq a was redundant. This is fixed now, along with
some simplification in Note [Replacement vs keeping]. There's
a tiny bit of extra complexity in setImplicationStatus, but
it's explained in Note [Tracking redundant constraints].
Close #20602
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PHASE 1: we never rewrite Concrete# evidence.
This patch migrates all the representation polymorphism checks to
the typechecker, using a new constraint form
Concrete# :: forall k. k -> TupleRep '[]
Whenever a type `ty` must be representation-polymorphic
(e.g. it is the type of an argument to a function), we emit a new
`Concrete# ty` Wanted constraint. If this constraint goes
unsolved, we report a representation-polymorphism error to the user.
The 'FRROrigin' datatype keeps track of the context of the
representation-polymorphism check, for more informative error messages.
This paves the way for further improvements, such as
allowing type families in RuntimeReps and improving the soundness
of typed Template Haskell. This is left as future work (PHASE 2).
fixes #17907 #20277 #20330 #20423 #20426
updates haddock submodule
-------------------------
Metric Decrease:
T5642
-------------------------
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We define Reduction = Reduction Coercion !Type.
A reduction of the form 'Reduction co new_ty' witnesses an
equality ty ~co~> new_ty.
That is, the rewriting happens left-to-right: the right-hand-side
type of the coercion is the rewritten type, and the left-hand-side
type the original type.
Sticking to this convention makes the codebase more consistent,
helping to avoid certain applications of SymCo.
This replaces the parts of the codebase which represented reductions as
pairs, (Coercion,Type) or (Type,Coercion).
Reduction being strict in the Type argument improves performance
in some programs that rewrite many type families (such as T9872).
Fixes #20161
-------------------------
Metric Decrease:
T5321Fun
T9872a
T9872b
T9872c
T9872d
-------------------------
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This commit expands the old Note [Type variable cycles in Givens] to apply
as well to Deriveds. See the Note for details and examples. This fixes a
regression introduced by my earlier commit that killed off the flattener in
favor of the rewriter.
A few other things happened along the way:
* unifyTest was renamed to touchabilityTest, because that's what it does.
* isInsolubleOccursCheck was folded into checkTypeEq, which does much of the
same work. To get this to work out, though, we need to keep more careful
track of what errors we spot in checkTypeEq, and so CheckTyEqResult has
become rather more glorious.
* A redundant Note or two was eliminated.
* Kill off occCheckForErrors; due to Note [Rewriting synonyms], the
extra occCheckExpand here is always redundant.
* Store blocked equalities separately from other inerts; less stuff
to look through when kicking out.
Close #19682.
test case: typecheck/should_compile/T19682{,b}
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This creates new modules GHC.Tc.Solver.InertSet and
GHC.Tc.Solver.Types. The Monad module is still pretty
big, but this is an improvement. Moreover, it means
that GHC.HsToCore.Pmc.Solver.Types no longer depends
on the constraint solver (it now depends on GHC.Tc.Solver.InertSet),
making the error-messages work easier.
This patch thus contributes to #18516.
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