[3.9] bpo-40334: Produce better error messages on invalid targets (GH-20106) (GH-20973)

* bpo-40334: Produce better error messages on invalid targets (GH-20106)

The following error messages get produced:
- `cannot delete ...` for invalid `del` targets
- `... is an illegal 'for' target` for invalid targets in for
  statements
- `... is an illegal 'with' target` for invalid targets in
  with statements

Additionally, a few `cut`s were added in various places before the
invocation of the `invalid_*` rule, in order to speed things
up.

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>
(cherry picked from commit 01ece63d42)

Lib/test/test_syntax.py

@@ -160,6 +160,61 @@
 Traceback (most recent call last):
 SyntaxError: 'list' is an illegal expression for augmented assignment
 
+Invalid targets in `for` loops and `with` statements should also
+produce a specialized error message
+
+>>> for a() in b: pass
+Traceback (most recent call last):
+SyntaxError: cannot assign to function call
+
+>>> for (a, b()) in b: pass
+Traceback (most recent call last):
+SyntaxError: cannot assign to function call
+
+>>> for [a, b()] in b: pass
+Traceback (most recent call last):
+SyntaxError: cannot assign to function call
+
+>>> for (*a, b, c+1) in b: pass
+Traceback (most recent call last):
+SyntaxError: cannot assign to operator
+
+>>> for (x, *(y, z.d())) in b: pass
+Traceback (most recent call last):
+SyntaxError: cannot assign to function call
+
+>>> for a, b() in c: pass
+Traceback (most recent call last):
+SyntaxError: cannot assign to function call
+
+>>> for i < (): pass
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+
+>>> with a as b(): pass
+Traceback (most recent call last):
+SyntaxError: cannot assign to function call
+
+>>> with a as (b, c()): pass
+Traceback (most recent call last):
+SyntaxError: cannot assign to function call
+
+>>> with a as [b, c()]: pass
+Traceback (most recent call last):
+SyntaxError: cannot assign to function call
+
+>>> with a as (*b, c, d+1): pass
+Traceback (most recent call last):
+SyntaxError: cannot assign to operator
+
+>>> with a as (x, *(y, z.d())): pass
+Traceback (most recent call last):
+SyntaxError: cannot assign to function call
+
+>>> with a as b, c as d(): pass
+Traceback (most recent call last):
+SyntaxError: cannot assign to function call
+
 >>> p = p =
 Traceback (most recent call last):
 SyntaxError: invalid syntax
@@ -736,7 +791,7 @@ def test_assign_del(self):
         self._check_error("del (1, 2)", "delete literal")
         self._check_error("del None", "delete None")
         self._check_error("del *x", "delete starred")
-        self._check_error("del (*x)", "delete starred")
+        self._check_error("del (*x)", "use starred expression")
         self._check_error("del (*x,)", "delete starred")
         self._check_error("del [*x,]", "delete starred")
         self._check_error("del f()", "delete function call")