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Previously, `can_eq_nc'` would equate `ForAllTy`s regardless of their
`ArgFlag`, including `forall i -> i -> Type` and `forall i. i -> Type`! To fix
this, `can_eq_nc'` now uses the `sameVis` function to first check if the
`ArgFlag`s are equal modulo specificity. I have also updated `tcEqType`'s
implementation to match this behavior. For more explanation on the "modulo
specificity" part, see the new `Note [ForAllTy and typechecker equality]`
in `GHC.Tc.Solver.Canonical`.
While I was in town, I fixed some related documentation issues:
* I added `Note [Typechecker equality]` to `GHC.Tc.Utils.TcType` to describe
what exactly distinguishes `can_eq_nc'` and `tcEqType` (which implement
typechecker equality) from `eqType` (which implements definitional equality,
which does not care about the `ArgFlags` of `ForAllTy`s at all).
* The User's Guide had some outdated prose on the specified/inferred
distinction being different for types and kinds, a holdover from #15079. This
is no longer the case on today's GHC, so I removed this prose, added some new
prose to take its place, and added a regression test for the programs in
#15079.
* The User's Guide had some _more_ outdated prose on inferred type variables
not being allowed in `default` type signatures for class methods, which is no
longer true as of the resolution of #18432.
* The related `Note [Deferred Unification]` was being referenced as
`Note [Deferred unification]` elsewhere, which made it harder to `grep`
for. I decided to change the name of the Note to `Deferred unification`
for consistency with the capitalization style used for most other Notes.
Fixes #18863.
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