gh-138716: Fix assert a := b syntax error message (#138718)

Author: sobolevn

Grammar/python.gram

@@ -1315,6 +1315,14 @@ invalid_assert_stmt:
             a, b,
             "cannot assign to %s here. Maybe you meant '==' instead of '='?",
             _PyPegen_get_expr_name(a)) }
+    | 'assert' a=expression ':=' b=expression {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(
+            a, b,
+            "cannot use named expression without parentheses here") }
+    | 'assert' expression ',' a=expression ':=' b=expression {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(
+            a, b,
+            "cannot use named expression without parentheses here") }
 invalid_block:
     | NEWLINE !INDENT { RAISE_INDENTATION_ERROR("expected an indented block") }
 invalid_comprehension:

Lib/test/test_syntax.py

@@ -2690,10 +2690,15 @@ def f(x: *b)
 Asserts:
 
     >>> assert (a := 1)  # ok
-    >>> # TODO(@sobolevn): improve this message in the next PR
+    >>> assert 1, (a := 1)  # ok
+
     >>> assert a := 1
     Traceback (most recent call last):
-    SyntaxError: invalid syntax
+    SyntaxError: cannot use named expression without parentheses here
+
+    >>> assert 1, a := 1
+    Traceback (most recent call last):
+    SyntaxError: cannot use named expression without parentheses here
 
     >>> assert 1 = 2 = 3
     Traceback (most recent call last):

Misc/NEWS.d/next/Core_and_Builtins/2025-09-09-23-59-13.gh-issue-138716.UawDY0.rst

@@ -0,0 +1,2 @@
+Improve :exc:`SyntaxError` message for :keyword:`assert` in cases like
+``assert a := b``.