gh-90994: Improve error messages upon call arguments syntax errors

Grammar/python.gram

@@ -957,6 +957,7 @@ kwargs[asdl_seq*]:
     | ','.kwarg_or_double_starred+
 
 starred_expression[expr_ty]:
+    | invalid_starred_expression
     | '*' a=expression { _PyAST_Starred(a, Load, EXTRA) }
 
 kwarg_or_starred[KeywordOrStarred*]:
@@ -1083,6 +1084,8 @@ invalid_arguments:
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, _PyPegen_get_last_comprehension_item(PyPegen_last_item(b, comprehension_ty)), "Generator expression must be parenthesized") }
     | a=NAME b='=' expression for_if_clauses {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '==' or ':=' instead of '='?")}
+    | (args ',')? a=NAME b='=' &(',' | ')') {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected argument value expression")}
     | a=args b=for_if_clauses { _PyPegen_nonparen_genexp_in_call(p, a, b) }
     | args ',' a=expression b=for_if_clauses {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, _PyPegen_get_last_comprehension_item(PyPegen_last_item(b, comprehension_ty)), "Generator expression must be parenthesized") }
@@ -1095,6 +1098,8 @@ invalid_kwarg:
     | !(NAME '=') a=expression b='=' {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(
             a, b, "expression cannot contain assignment, perhaps you meant \"==\"?") }
+    | a='**' expression '=' b=expression {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "cannot assign to keyword argument unpacking") }
 
 # IMPORTANT: Note that the "_without_invalid" suffix causes the rule to not call invalid rules under it
 expression_without_invalid[expr_ty]:
@@ -1328,3 +1333,5 @@ invalid_kvpair:
         RAISE_ERROR_KNOWN_LOCATION(p, PyExc_SyntaxError, a->lineno, a->end_col_offset - 1, a->end_lineno, -1, "':' expected after dictionary key") }
     | expression ':' a='*' bitwise_or { RAISE_SYNTAX_ERROR_STARTING_FROM(a, "cannot use a starred expression in a dictionary value") }
     | expression a=':' &('}'|',') {RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expression expected after dictionary key and ':'") }
+invalid_starred_expression:
+    | a='*' expression '=' b=expression { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "cannot assign to iterable argument unpacking") }

Lib/test/test_syntax.py

@@ -756,6 +756,27 @@
 >>> __debug__: int
 Traceback (most recent call last):
 SyntaxError: cannot assign to __debug__
+>>> f(a=)
+Traceback (most recent call last):
+SyntaxError: expected argument value expression
+>>> f(a, b, c=)
+Traceback (most recent call last):
+SyntaxError: expected argument value expression
+>>> f(a, b, c=, d)
+Traceback (most recent call last):
+SyntaxError: expected argument value expression
+>>> f(*args=[0])
+Traceback (most recent call last):
+SyntaxError: cannot assign to iterable argument unpacking
+>>> f(a, b, *args=[0])
+Traceback (most recent call last):
+SyntaxError: cannot assign to iterable argument unpacking
+>>> f(**kwargs={'a': 1})
+Traceback (most recent call last):
+SyntaxError: cannot assign to keyword argument unpacking
+>>> f(a, b, *args, **kwargs={'a': 1})
+Traceback (most recent call last):
+SyntaxError: cannot assign to keyword argument unpacking
 
 
 More set_context():

Misc/NEWS.d/next/Core and Builtins/2022-09-17-17-08-01.gh-issue-90994.f0H2Yd.rst

@@ -0,0 +1,4 @@
+Improve error messages when there's a syntax error with call arguments. The following three cases are covered:
+- No value is assigned to a named argument, eg ``foo(a=)``.
+- A value is assigned to a star argument, eg ``foo(*args=[0])``.
+- A value is assigned to a double-star keyword argument, eg ``foo(**kwarg={'a': 0})``.