Python: Update python.tsg

First, we extend the various location overriding hacks to also accept
list and dict splats in various places. Having done this, we then have
to tackle how to actually desugar these new comprehension forms (as this
is what we currently do for the old forms).

As a reminder, a list comprehension like `[x for x in y]` currently gets
desugared into a small local function, something like

```python
def listcomp(a):
    for x in a:
        yield x
listcomp(y)
```

For `[*x for x in y]`, the behaviour we want is that we unpack `x`
before yielding its elements in turn. This is essentially what we would
get if we were to use `yield from x` instead of `yield x` in the above
desugaring, so that's what we do. This also works for set
comprehensions.

For dict comprehensions, it's slightly more complicated. Here, the
generator function instead yields a stream of `(key, value)` tuples.
(And apparently the old parser got this wrong and emitted `(value, key)`
pairs instead, which we faithfully recreated in the new parser as well.
We fix that bug in both parsers while we're at it). So, a bare `yield
from` is not enough, we also need a `.items()` call to get the
double-starred expression to emit its items as a stream of tuples (that
we then `yield from`.

To make this (hopefully) less verbose in the implementation, we defer
the decision of whether to use `yield` or `yield from` by introducing a
`yield_kind` scoped variable that determines the type of the actual AST
node. And of course for dict comprehensions with unpacking we need to
synthesise the extra machinery mentioned above.

On the plus side, this means we don't have to mess with control-flow, as
the existing machinery should be able to handle the desugared syntax
just fine.

python/extractor/tsg-python/python.tsg

@@ -403,7 +403,7 @@
 
 ;;; GeneratorExp
 
-(generator_expression . "(" . (comment)* . (expression) @start [(for_in_clause) (if_clause)] @end . (comment)* . ")" .) @generatorexp
+(generator_expression . "(" . (comment)* . [(expression) (list_splat)] @start [(for_in_clause) (if_clause)] @end . (comment)* . ")" .) @generatorexp
 {
     attr (@generatorexp.node) _location_start = (location-start @start)
     attr (@generatorexp.node) _location_end = (location-end @end)
@@ -415,13 +415,13 @@
     attr (@if.node) _location_end = (location-end @expr)
 }
 
-(generator_expression . "(" . (comment)* . (expression) @start (for_in_clause) @child [(for_in_clause) (if_clause)] @end . (comment)* . ")" .) @_genexpr
+(generator_expression . "(" . (comment)* . [(expression) (list_splat)] @start (for_in_clause) @child [(for_in_clause) (if_clause)] @end . (comment)* . ")" .) @_genexpr
 {
     attr (@child.node) _location_start = (location-start @start)
     attr (@child.node) _location_end = (location-end @end)
 }
 
-(generator_expression . "(" . (comment)* . (expression) @start (for_in_clause) @end . (comment)* . ")" .) @_genexpr
+(generator_expression . "(" . (comment)* . [(expression) (list_splat)] @start (for_in_clause) @end . (comment)* . ")" .) @_genexpr
 {
     attr (@end.node) _location_start = (location-start @start)
     attr (@end.node) _location_end = (location-end @end)
@@ -824,6 +824,29 @@
     attr (@genexpr.arg_use) ctx = "load"
 }
 
+; DictComp with unpacking (PEP 798): `{**d for d in dicts}`
+(dictionary_comprehension
+    body: (dictionary_splat)
+) @genexpr
+{
+    let @genexpr.fun = (ast-node @genexpr "Function")
+    attr (@genexpr.node) function = @genexpr.fun
+    attr (@genexpr.fun) name = "dictcomp"
+
+    let @genexpr.arg = (ast-node @genexpr "Name")
+    attr (@genexpr.arg) variable = ".0"
+    attr (@genexpr.arg) ctx = "param"
+
+    edge @genexpr.fun -> @genexpr.arg
+    attr (@genexpr.fun -> @genexpr.arg) args = 0
+    attr (@genexpr.fun) kwonlyargs = #null
+    attr (@genexpr.fun) kwarg = #null
+
+    let @genexpr.arg_use = (ast-node @genexpr "Name")
+    attr (@genexpr.arg_use) variable = ".0"
+    attr (@genexpr.arg_use) ctx = "load"
+}
+
 ;;;;;; End of DictComp (`{a: b for c in d if e}`)
 
 ;;;;;; GeneratorExp (`(a for b in c if d)`)
@@ -862,7 +885,7 @@
 ; information for the entire generator expression (yes, it is a wide parameter!) and so we must recreate the logic for
 ; setting this location information correctly.
 
-(generator_expression . "(" . (comment)* . (expression) @start  [(for_in_clause) (if_clause)] @end . (comment)* . ")" .) @genexpr
+(generator_expression . "(" . (comment)* . [(expression) (list_splat)] @start  [(for_in_clause) (if_clause)] @end . (comment)* . ")" .) @genexpr
 {
     ; Synthesize the `genexpr` function
     let @genexpr.fun = (ast-node @genexpr "Function")
@@ -1034,12 +1057,25 @@
 ; For everything except dictionary comprehensions, the innermost expression is just the `body` of the
 ; comprehension.
 [
-    (generator_expression body: (_) @body) @genexpr
-    (list_comprehension body: (_) @body) @genexpr
-    (set_comprehension body: (_) @body) @genexpr
+    (generator_expression body: (expression) @body) @genexpr
+    (list_comprehension body: (expression) @body) @genexpr
+    (set_comprehension body: (expression) @body) @genexpr
 ]
 {
     let @genexpr.result = @body.node
+    let @genexpr.yield_kind = "Yield"
+}
+
+; For starred comprehensions (PEP 798), the result is the inner expression (not the Starred
+; wrapper), and we use `yield from` instead of `yield` to represent the unpacking semantics.
+[
+    (generator_expression body: (list_splat (expression) @inner) @_body) @genexpr
+    (list_comprehension body: (list_splat (expression) @inner) @_body) @genexpr
+    (set_comprehension body: (list_splat (expression) @inner) @_body) @genexpr
+]
+{
+    let @genexpr.result = @inner.node
+    let @genexpr.yield_kind = "YieldFrom"
 }
 
 ; For dict comprehensions, we build an explicit tuple using the key and value pair.
@@ -1052,13 +1088,31 @@
 {
     let tuple = (ast-node @body "Tuple")
     edge tuple -> @key.node
-    attr (tuple -> @key.node) elts = 1
+    attr (tuple -> @key.node) elts = 0
     edge tuple -> @value.node
-    attr (tuple -> @value.node) elts = 0
-    ; TODO verify that it is correct to use a `(value, key)` tuple, and not a `(key, value)` tuple above.
-    ; That is what the current parser does...
+    attr (tuple -> @value.node) elts = 1
     attr (tuple) ctx = "load"
     let @genexpr.result = tuple
+    let @genexpr.yield_kind = "Yield"
+}
+
+; For dict comprehensions with unpacking (PEP 798), `{**d for d in dicts}` desugars to
+; `yield from d.items()` to produce (key, value) tuples consistent with the regular dict comp model.
+(dictionary_comprehension
+    body: (dictionary_splat (expression) @inner) @_body
+) @genexpr
+{
+    ; Synthesize `d.items()`: Attribute(value=d, attr='items') then Call(func=attr)
+    let attr = (ast-node @inner "Attribute")
+    attr (attr) value = @inner.node
+    attr (attr) attr = "items"
+    attr (attr) ctx = "load"
+
+    let call = (ast-node @inner "Call")
+    attr (call) func = attr
+
+    let @genexpr.result = call
+    let @genexpr.yield_kind = "YieldFrom"
 }
 
 ; For the final clause, we need to hook it up with the rest of the expression.
@@ -1094,7 +1148,7 @@
     let last = (get-last-element @last_candidates)
 
     let expr = (ast-node @body "Expr")
-    let yield = (ast-node @body "Yield")
+    let yield = (ast-node @body @genexpr.yield_kind)
 
     let @genexpr.expr = expr
     let @genexpr.yield = yield