This previously only worked in certain circumstances. In particular,
assignments such as `match[1] = ...` or even just `match[1]` would fail
to parse correctly.
Fixing this turned out to be less trivial than anticipated. Consider the
fact that
```
match [1]: case (...)
```
can either look the start of a `match` statement, or it could be a type
ascription, ascribing the value of `case(...)` (a call) to the item at
index 1 of `match`.
To fix this, then, we give `match` the identifier and `match` the
statement the same precendence in the grammar, and additionally also
mark a conflict between `match_statement` and `primary_expression`. This
causes the conflict to be resolved dynamically, and seems to do the
right thing in all cases.
Once again, the interaction between anchors and extras (specifically
comments) was causing trouble.
The root of the problem was the fact that in `a[b]`, we put `b` in the
`index` field of the subscript node, whereas in `a[b,c]`, we
additionally synthesize a `Tuple` node for `b,c` (which matches the
Python AST).
To fix this, we refactored the grammar slightly so as to make that tuple
explicit, such that a subscript node either contains a single expression
or the newly added tuple node. This greatly simplifies the logic.
Found when parsing `Lib/test/test_coroutines.py` using the new parser.
For whatever reason, having `await` be an `expression` (with an argument
of the same kind) resulted in a bad parse. Consulting the official
grammar, we see that `await` should actually be a `primary_expression`
instead. This is also more in line with the other unary operators, whose
precedence is shared by the `await` syntax.
Turns out, `except*` is actually not a token on its own according to the
Python grammar. This means it's legal to write `except *foo: ...`, which
we previously would consider a syntax error.
To fix it, we simply break up the `except*` into two separate tokens.
That is, the `*T` in `def foo(*args : *T): ...`.
This is apparently a piece of syntax we did not support correctly until
now.
In terms of the grammar, we simply add `list_splat` as a possible
alternative for `type` (which could previously only be an `expression`).
We also update `python.tsg` to not specify `expression` those places (as
the relevant stanzas will then not work for `list_splat`s).
This syntax is not supported by the old parser, hence we only add a new
parser test for it.
