gh-91210: Improve error message when non-default param follows default (GH-95933)

- Improve error message when parameter without a default follows one with a default
- Show same error message when positional-only params precede the default/non-default sequence

2 files changed, 29 insertions, 12 deletions

diff --git a/Lib/test/test_positional_only_arg.py b/Lib/test/test_positional_only_arg.py
index 0a9503e..1a19381 100644
--- a/Lib/test/test_positional_only_arg.py
+++ b/Lib/test/test_positional_only_arg.py
@@ -23,10 +23,11 @@ class PositionalOnlyTestCase(unittest.TestCase):
             compile(codestr + "\n", "<test>", "single")
 
     def test_invalid_syntax_errors(self):
-        check_syntax_error(self, "def f(a, b = 5, /, c): pass", "non-default argument follows default argument")
-        check_syntax_error(self, "def f(a = 5, b, /, c): pass", "non-default argument follows default argument")
-        check_syntax_error(self, "def f(a = 5, b=1, /, c, *, d=2): pass", "non-default argument follows default argument")
-        check_syntax_error(self, "def f(a = 5, b, /): pass", "non-default argument follows default argument")
+        check_syntax_error(self, "def f(a, b = 5, /, c): pass", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "def f(a = 5, b, /, c): pass", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "def f(a = 5, b=1, /, c, *, d=2): pass", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "def f(a = 5, b, /): pass", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "def f(a, /, b = 5, c): pass", "parameter without a default follows parameter with a default")
         check_syntax_error(self, "def f(*args, /): pass")
         check_syntax_error(self, "def f(*args, a, /): pass")
         check_syntax_error(self, "def f(**kwargs, /): pass")
@@ -44,10 +45,11 @@ class PositionalOnlyTestCase(unittest.TestCase):
         check_syntax_error(self, "def f(a, *, c, /, d, e): pass")
 
     def test_invalid_syntax_errors_async(self):
-        check_syntax_error(self, "async def f(a, b = 5, /, c): pass", "non-default argument follows default argument")
-        check_syntax_error(self, "async def f(a = 5, b, /, c): pass", "non-default argument follows default argument")
-        check_syntax_error(self, "async def f(a = 5, b=1, /, c, d=2): pass", "non-default argument follows default argument")
-        check_syntax_error(self, "async def f(a = 5, b, /): pass", "non-default argument follows default argument")
+        check_syntax_error(self, "async def f(a, b = 5, /, c): pass", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "async def f(a = 5, b, /, c): pass", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "async def f(a = 5, b=1, /, c, d=2): pass", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "async def f(a = 5, b, /): pass", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "async def f(a, /, b = 5, c): pass", "parameter without a default follows parameter with a default")
         check_syntax_error(self, "async def f(*args, /): pass")
         check_syntax_error(self, "async def f(*args, a, /): pass")
         check_syntax_error(self, "async def f(**kwargs, /): pass")
@@ -231,9 +233,11 @@ class PositionalOnlyTestCase(unittest.TestCase):
         self.assertEqual(x(1, 2), 3)
 
     def test_invalid_syntax_lambda(self):
-        check_syntax_error(self, "lambda a, b = 5, /, c: None", "non-default argument follows default argument")
-        check_syntax_error(self, "lambda a = 5, b, /, c: None", "non-default argument follows default argument")
-        check_syntax_error(self, "lambda a = 5, b, /: None", "non-default argument follows default argument")
+        check_syntax_error(self, "lambda a, b = 5, /, c: None", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "lambda a = 5, b, /, c: None", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "lambda a = 5, b=1, /, c, *, d=2: None", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "lambda a = 5, b, /: None", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "lambda a, /, b = 5, c: None", "parameter without a default follows parameter with a default")
         check_syntax_error(self, "lambda *args, /: None")
         check_syntax_error(self, "lambda *args, a, /: None")
         check_syntax_error(self, "lambda **kwargs, /: None")
diff --git a/Lib/test/test_syntax.py b/Lib/test/test_syntax.py
index ae10669..69a8623 100644
--- a/Lib/test/test_syntax.py
+++ b/Lib/test/test_syntax.py
@@ -334,7 +334,12 @@ From ast_for_arguments():
 >>> def f(x, y=1, z):
 ...     pass
 Traceback (most recent call last):
-SyntaxError: non-default argument follows default argument
+SyntaxError: parameter without a default follows parameter with a default
+
+>>> def f(x, /, y=1, z):
+...     pass
+Traceback (most recent call last):
+SyntaxError: parameter without a default follows parameter with a default
 
 >>> def f(x, None):
 ...     pass
@@ -555,6 +560,14 @@ SyntaxError: expected default value expression
 Traceback (most recent call last):
 SyntaxError: expected default value expression
 
+>>> lambda a,d=3,c: None
+Traceback (most recent call last):
+SyntaxError: parameter without a default follows parameter with a default
+
+>>> lambda a,/,d=3,c: None
+Traceback (most recent call last):
+SyntaxError: parameter without a default follows parameter with a default
+
 >>> import ast; ast.parse('''
 ... def f(
 ...     *, # type: int