gh-138864: Improve SyntaxError message for case with expression

Grammar/python.gram

@@ -495,6 +495,7 @@ case_block[match_case_ty]:
     | invalid_case_block
     | "case" pattern=patterns guard=guard? ':' body=block {
         _PyAST_match_case(pattern, guard, body, p->arena) }
+    | invalid_case_pattern
 
 guard[expr_ty]: 'if' guard=named_expression { guard }
 
@@ -1480,6 +1481,10 @@ invalid_case_block:
     | "case" patterns guard? NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
     | a="case" patterns guard? ':' NEWLINE !INDENT {
         RAISE_INDENTATION_ERROR("expected an indented block after 'case' statement on line %d", a->lineno) }
+invalid_case_pattern:
+    | "case" a=expression guard? ':' block {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
+            a, "cannot use %s as case pattern", _PyPegen_get_expr_name(a)) }
 invalid_as_pattern:
     | or_pattern 'as' a="_" { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot use '_' as a target") }
     | or_pattern 'as' a=expression {

Lib/test/test_exceptions.py

@@ -248,7 +248,7 @@ def testSyntaxErrorOffset(self):
         check('[\nfile\nfor str(file)\nin\n[]\n]', 3, 5)
         check('[file for\n str(file) in []]', 2, 2)
         check("ages = {'Alice'=22, 'Bob'=23}", 1, 9)
-        check('match ...:\n    case {**rest, "key": value}:\n        ...', 2, 19)
+        check('match ...:\n    case {**rest, "key": value}:\n        ...', 2, 10)
         check("[a b c d e f]", 1, 2)
         check("for x yfff:", 1, 7)
         check("f(a for a in b, c)", 1, 3, 1, 15)

Lib/test/test_syntax.py

@@ -374,13 +374,38 @@
 ...     case {**rest, "key": value}:
 ...        ...
 Traceback (most recent call last):
-SyntaxError: invalid syntax
+SyntaxError: cannot use dict literal as case pattern
 
 >>> match ...:
 ...     case {**_}:
 ...        ...
 Traceback (most recent call last):
-SyntaxError: invalid syntax
+SyntaxError: cannot use dict literal as case pattern
+
+>>> match ...:
+...    case ...: ...
+Traceback (most recent call last):
+SyntaxError: cannot use ellipsis as case pattern
+
+>>> match ...:
+...    case 1 // 2: ...
+Traceback (most recent call last):
+SyntaxError: cannot use expression as case pattern
+
+>>> match ...:
+...    case {1, 2, 3}: ...
+Traceback (most recent call last):
+SyntaxError: cannot use set display as case pattern
+
+>>> match ...:
+...    case a[0]: ...
+Traceback (most recent call last):
+SyntaxError: cannot use subscript as case pattern
+
+>>> match ...:
+...    case a[0].method(): ...
+Traceback (most recent call last):
+SyntaxError: cannot use function call as case pattern
 
 # But prefixes of soft keywords should
 # still raise specialized errors

Misc/NEWS.d/next/Core_and_Builtins/2025-09-13-12-51-34.gh-issue-138864.Otm5li.rst

@@ -0,0 +1,2 @@
+Improve :exc:`SyntaxError` message for using ``case``
+with invalid patterns.