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Previously, associated type family defaults were validity-checked
during typechecking. Unfortunately, the error messages that these
checks produce run the risk of printing knot-tied type constructors,
which will cause GHC to diverge. In order to preserve the current
error message's descriptiveness, this patch postpones these validity
checks until after typechecking, which are now located in the new
function `GHC.Tc.Validity.checkValidAssocTyFamDeflt`.
Fixes #18648.
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Associated type family default declarations behave strangely in a
couple of ways:
1. If one tries to bind the type variables with an explicit `forall`,
the `forall`'d part will simply be ignored. (#16110)
2. One cannot use visible kind application syntax on the left-hand
sides of associated default equations, unlike every other form
of type family equation. (#16356)
Both of these issues have a common solution. Instead of using
`LHsQTyVars` to represent the left-hand side arguments of an
associated default equation, we instead use `HsTyPats`, which is what
other forms of type family equations use. In particular, here are
some highlights of this patch:
* `FamEqn` is no longer parameterized by a `pats` type variable, as
the `feqn_pats` field is now always `HsTyPats`.
* The new design for `FamEqn` in chronicled in
`Note [Type family instance declarations in HsSyn]`.
* `TyFamDefltEqn` now becomes the same thing as `TyFamInstEqn`. This
means that many of `TyFamDefltEqn`'s code paths can now reuse the
code paths for `TyFamInstEqn`, resulting in substantial
simplifications to various parts of the code dealing with
associated type family defaults.
Fixes #16110 and #16356.
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