We use `nullKind` for the `group` parser combinator.
When pattern matching `nullKind` nodes, we check their arities.
So, error recovery often fails for parsers that use the `group`
combinator.
For example, we have the parser
```
def whereDecls := leading_parser "where " >> many1Indent (group (letRecDecl >> optional ";"))
```
If there is syntax error at `letRecDecl`, the node corresponding to
```
group (letRecDecl >> optional ";")
```
will contain only one child, and the pattern matching at
```
def expandWhereDecls (whereDecls : Syntax) (body : Syntax) : MacroM Syntax :=
match whereDecls with
| `(whereDecls|where $[$decls:letRecDecl $[;]?]*) => `(let rec $decls:letRecDecl,*; $body)
| _ => Macro.throwUnsupported
```
fails, and we can't elaborate the partial syntax tree for
`letRecDecl`, and auto-completion will not work there.
We address this issue by using a new kind for the `group` combinator.
The idea is to pattern match `group` as we pattern match `node`s with
proper syntax node kinds. This change is consistent with the way we
use `group` where it mainly a convenience for saving us the trouble of
defining a new parser definition that is used only once.
@Kha I had some unexpected surprises, but it is a good change.
Here is the summary.
1- We could get rid of `a %ₙ b` and `ModN` class. We can use `HMod`
instead. It was a positive surprise since I didn't remember we had
this `ModN` class.
2- Coercions are never used in heterogeneous operators. This is
expected since `a * b` is now notation for `HMul.hMul a b`, and
`a` and `b` may have different types. I manually added instances such
as `HMul Nat Int Int`. However, I did not try to add generic instances
such as
```
instance [Coe a b] [Mul b] : HMul a b b where
hMul x y := mul (coe x) y
```
I will try later.
3- Give `h : cs.size > 0`, I got a type error at
```
let idx : Fin cs.size := ⟨cs.size - 1, Nat.predLt h⟩
```
`Nat.predLt h` has type `Nat.pred cs.size < cs.size`
However, `Nat.pred cs.size` doesn't unify with `cs.size - 1`.
The problem is that we can't synthesize the `HSub` instance until
we apply the default instances.
It worked before because `isDefEq` would force the pending TC
problem `Sub Nat` to be resolved, and after that we would be able
to reduce `cs.size - 1` and establish that it is definitionally
equal to `Nat.pred cs.size`.
I considered two possible workarounds
a) `let idx : Fin cs.size := ⟨cs.size - (1:Nat), Nat.predLt h⟩`
b) `let idx : Fin cs.size := ⟨cs.size - 1, by exact Nat.predLt h⟩`
The first one works because we are not providing enough information
for synthesizing the `HSub` instance. The second works because it
postpones the elaboration of `Nat.predLt h`. The default instances
will be applied before we start applying tactics.
4- The `.` notation is affected too. For example, `(x + 1).toUInt8`
doesn't work since we don't know the type of `x+1` until we apply
default instances. I fixed it by using `(x + (1:Nat)).toUInt8`.
Another possible fix is `Nat.toUInt8 (x + 1)`.
Similarly, `(x+1).fold ...` doesn't work.
5- The following code failed to be elaborated
```
indent (push s!"{ss'}\n") (some (0 - Format.getIndent (← getOptions)))
```
It was working before, but it relied on how the expected type is
propagated. The elaborator process
```
some (0 - Format.getIndent (← getOptions))
```
with expected type `(Option Int)`. So, the `-` is interpreted as
`Int.sub` although `Format.getIndent (← getOptions)` has type `Nat`.
In the new `HSub`, the expected type doesn't really influence TC
resolution since it is an `outparam`. So, we failed with the error
failed to synthesize `HSub Nat Nat Int`.
One possible fix was to add the instance `HSub Nat Nat Int` with
`Int.sub`, but I used the following fix
```
some ((0 : Int) - Format.getIndent (← getOptions))
```
which makes it clear that we want the `Int.sub` operator instead of
`Nat.sub`.
