gh-123562: Improve SyntaxError message for case ... as a.b (#123563)

Lib/test/test_patma.py

@@ -3015,6 +3015,13 @@ class TestSyntaxErrors(unittest.TestCase):
                 pass
         """)
 
+    def test_multiple_assignments_to_name_in_pattern_6(self):
+        self.assert_syntax_error("""
+        match ...:
+            case a as a + 1:  # NAME and expression with no ()
+                pass
+        """)
+
     def test_multiple_starred_names_in_sequence_pattern_0(self):
         self.assert_syntax_error("""
         match ...:

Lib/test/test_syntax.py

@@ -1932,7 +1932,31 @@ Corner-cases that used to crash:
     ...   case 42 as 1+2+4:
     ...     ...
     Traceback (most recent call last):
-    SyntaxError: invalid pattern target
+    SyntaxError: cannot use expression as pattern target
+
+    >>> match ...:
+    ...   case 42 as a.b:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use attribute as pattern target
+
+    >>> match ...:
+    ...   case 42 as (a, b):
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use tuple as pattern target
+
+    >>> match ...:
+    ...   case 42 as (a + 1):
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use expression as pattern target
+
+    >>> match ...:
+    ...   case (32 as x) | (42 as a()):
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use function call as pattern target
 
     >>> match ...:
     ...   case Foo(z=1, y=2, x):
@@ -2817,6 +2841,22 @@ while 1:
             end_offset=22 + len("obj.attr"),
         )
 
+    def test_match_stmt_invalid_as_expr(self):
+        self._check_error(
+            textwrap.dedent(
+                """
+                match 1:
+                    case x as obj.attr:
+                        ...
+                """
+            ),
+            errtext="cannot use attribute as pattern target",
+            lineno=3,
+            end_lineno=3,
+            offset=15,
+            end_offset=15 + len("obj.attr"),
+        )
+
 
 def load_tests(loader, tests, pattern):
     tests.addTest(doctest.DocTestSuite())