bpo-40903: Handle multiple '=' in invalid assignment rules in the PEG parser (GH-20697)

Automerge-Triggered-By: @pablogsal
(cherry picked from commit 9f495908c5)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

Grammar/python.gram

@@ -92,7 +92,7 @@ assignment[stmt_ty]:
     | a=('(' b=single_target ')' { b }
          | single_subscript_attribute_target) ':' b=expression c=['=' d=annotated_rhs { d }] {
         CHECK_VERSION(6, "Variable annotations syntax is", _Py_AnnAssign(a, b, c, 0, EXTRA)) }
-    | a=(z=star_targets '=' { z })+ b=(yield_expr | star_expressions) tc=[TYPE_COMMENT] {
+    | a=(z=star_targets '=' { z })+ b=(yield_expr | star_expressions) !'=' tc=[TYPE_COMMENT] {
          _Py_Assign(a, b, NEW_TYPE_COMMENT(p, tc), EXTRA) }
     | a=single_target b=augassign c=(yield_expr | star_expressions) {
          _Py_AugAssign(a, b->kind, c, EXTRA) }
@@ -646,10 +646,11 @@ invalid_assignment:
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "only single target (not tuple) can be annotated") }
     | a=expression ':' expression ['=' annotated_rhs] {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "illegal target for annotation") }
-    | a=star_expressions '=' (yield_expr | star_expressions) {
+    | (star_targets '=')* a=star_expressions '=' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
             _PyPegen_get_invalid_target(a),
             "cannot assign to %s", _PyPegen_get_expr_name(_PyPegen_get_invalid_target(a))) }
+    | (star_targets '=')* a=yield_expr '=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "assignment to yield expression not possible") }
     | a=star_expressions augassign (yield_expr | star_expressions) {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION( 
             a, 

Lib/test/test_syntax.py

@@ -63,6 +63,10 @@ SyntaxError: cannot assign to __debug__
 Traceback (most recent call last):
 SyntaxError: cannot assign to function call
 
+>>> yield = 1
+Traceback (most recent call last):
+SyntaxError: assignment to yield expression not possible
+
 >>> del f()
 Traceback (most recent call last):
 SyntaxError: cannot delete function call
@@ -136,6 +140,18 @@ SyntaxError: cannot assign to operator
 Traceback (most recent call last):
 SyntaxError: cannot assign to conditional expression
 
+>>> True = True = 3
+Traceback (most recent call last):
+SyntaxError: cannot assign to True
+
+>>> x = y = True = z = 3
+Traceback (most recent call last):
+SyntaxError: cannot assign to True
+
+>>> x = y = yield = 1
+Traceback (most recent call last):
+SyntaxError: assignment to yield expression not possible
+
 >>> a, b += 1, 2
 Traceback (most recent call last):
 SyntaxError: 'tuple' is an illegal expression for augmented assignment
@@ -148,6 +164,10 @@ SyntaxError: 'tuple' is an illegal expression for augmented assignment
 Traceback (most recent call last):
 SyntaxError: 'list' is an illegal expression for augmented assignment
 
+>>> p = p =
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+
 From compiler_complex_args():
 
 >>> def f(None=1):
@@ -155,7 +175,6 @@ From compiler_complex_args():
 Traceback (most recent call last):
 SyntaxError: invalid syntax
 
-
 From ast_for_arguments():
 
 >>> def f(x, y=1, z):

Misc/NEWS.d/next/Core and Builtins/2020-06-07-22-50-10.bpo-40903.7dWejS.rst

@@ -0,0 +1 @@
+Fixed a possible segfault in the new PEG parser when producing error messages for invalid assignments of the form :code:`p=p=`. Patch by Pablo Galindo

Parser/pegen/pegen.c

@@ -161,6 +161,7 @@ byte_offset_to_character_offset(PyObject *line, int col_offset)
 const char *
 _PyPegen_get_expr_name(expr_ty e)
 {
+    assert(e != NULL);
     switch (e->kind) {
         case Attribute_kind:
             return "attribute";