bpo-40334: Error message for invalid default args in function call (GH-19973)

When parsing something like `f(g()=2)`, where the name of a default arg
is not a NAME, but an arbitrary expression, a specialised error message
is emitted.

Author: lysnikolaou

Grammar/python.gram

@@ -548,10 +548,12 @@ kwarg_or_starred[KeywordOrStarred*]:
     | a=NAME '=' b=expression {
         _PyPegen_keyword_or_starred(p, CHECK(_Py_keyword(a->v.Name.id, b, EXTRA)), 1) }
     | a=starred_expression { _PyPegen_keyword_or_starred(p, a, 0) }
+    | invalid_kwarg
 kwarg_or_double_starred[KeywordOrStarred*]:
     | a=NAME '=' b=expression {
         _PyPegen_keyword_or_starred(p, CHECK(_Py_keyword(a->v.Name.id, b, EXTRA)), 1) }
     | '**' a=expression { _PyPegen_keyword_or_starred(p, CHECK(_Py_keyword(NULL, a, EXTRA)), 1) }
+    | invalid_kwarg
 
 # NOTE: star_targets may contain *bitwise_or, targets may not.
 star_targets[expr_ty]:
@@ -620,6 +622,8 @@ incorrect_arguments:
     | expression for_if_clauses ',' [args | expression for_if_clauses] {
         RAISE_SYNTAX_ERROR("Generator expression must be parenthesized") }
     | a=args ',' args { _PyPegen_arguments_parsing_error(p, a) }
+invalid_kwarg:
+    | expression '=' { RAISE_SYNTAX_ERROR("expression cannot contain assignment, perhaps you meant \"==\"?") }
 invalid_named_expression:
     | a=expression ':=' expression {
         RAISE_SYNTAX_ERROR("cannot use assignment expressions with %s", _PyPegen_get_expr_name(a)) }

Lib/test/test_exceptions.py

@@ -242,16 +242,16 @@ def baz():
         check('from __future__ import doesnt_exist', 1, 1)
         check('from __future__ import braces', 1, 1)
         check('x=1\nfrom __future__ import division', 2, 1)
+        check('(yield i) = 2', 1, 1)
         check('def f(*):\n  pass', 1, 7 if support.use_old_parser() else 8)
+        check('foo(1=2)', 1, 5 if support.use_old_parser() else 6)
 
     @support.skip_if_new_parser("Pegen column offsets might be different")
     def testSyntaxErrorOffsetCustom(self):
         self.check('for 1 in []: pass', 1, 5)
         self.check('[*x for x in xs]', 1, 2)
         self.check('def f():\n  x, y: int', 2, 3)
-        self.check('(yield i) = 2', 1, 1)
         self.check('foo(x for x in range(10), 100)', 1, 5)
-        self.check('foo(1=2)', 1, 5)
 
     @cpython_only
     def testSettingException(self):

Lib/test/test_peg_parser.py

@@ -609,6 +609,9 @@ def f():
     ("lambda *: pass", "named arguments must follow bare *"),
     ("lambda *,: pass", "named arguments must follow bare *"),
     ("lambda *, **a: pass", "named arguments must follow bare *"),
+    ("f(g()=2", "expression cannot contain assignment, perhaps you meant \"==\"?"),
+    ("f(a, b, *c, d.e=2", "expression cannot contain assignment, perhaps you meant \"==\"?"),
+    ("f(*a, **b, c=0, d[1]=3)", "expression cannot contain assignment, perhaps you meant \"==\"?"),
 ]
 
 GOOD_BUT_FAIL_TEST_CASES = [