gh-138857: Improve error message for case outside of match (#138858)

* gh-138857: Improve error message for `case` outside of `match`

---------

Co-authored-by: Bartosz Sławecki <[email protected]>

Author: sobolevn

Grammar/python.gram

@@ -1477,6 +1477,10 @@ invalid_match_stmt:
     | "match" subject_expr NEWLINE { CHECK_VERSION(void*, 10, "Pattern matching is", RAISE_SYNTAX_ERROR("expected ':'") ) }
     | a="match" subject=subject_expr ':' NEWLINE !INDENT {
         RAISE_INDENTATION_ERROR("expected an indented block after 'match' statement on line %d", a->lineno) }
+    | a="case" patterns guard? b=':' block {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(
+            a, b,
+            "case statement must be inside match statement") }
 invalid_case_block:
     | "case" patterns guard? NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
     | a="case" patterns guard? ':' NEWLINE !INDENT {

Lib/test/test_syntax.py

@@ -376,6 +376,42 @@
 Traceback (most recent call last):
 SyntaxError: invalid syntax
 
+# Check incorrect "case" placement with specialized error messages
+
+>>> case "pattern": ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> case 1 | 2: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> case klass(attr=1) | {}: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> case [] if x > 1: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> case match: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> case case: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> if some:
+...     case 1: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> case some:
+...     case 1: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
 # But prefixes of soft keywords should
 # still raise specialized errors
 

Misc/NEWS.d/next/Core_and_Builtins/2025-09-13-01-23-25.gh-issue-138857.YQ5gdc.rst

@@ -0,0 +1,2 @@
+Improve :exc:`SyntaxError` message for ``case`` keyword placed outside
+:keyword:`match` body.