gh-119724: Revert "bpo-45759: Better error messages for non-matching 'elif'/'else' statements (#29513)" (#119974)

This reverts commit 1c8f912ebd.

Grammar/python.gram

@@ -127,7 +127,6 @@ simple_stmt[stmt_ty] (memo):
     | &'nonlocal' nonlocal_stmt
 
 compound_stmt[stmt_ty]:
-    | invalid_compound_stmt
     | &('def' | '@' | 'async') function_def
     | &'if' if_stmt
     | &('class' | '@') class_def
@@ -1323,10 +1322,6 @@ invalid_import_from_targets:
     | token=NEWLINE { 
         RAISE_SYNTAX_ERROR_STARTING_FROM(token, "Expected one or more names after 'import'") }
 
-invalid_compound_stmt:
-    | a='elif' named_expression ':' { RAISE_SYNTAX_ERROR_STARTING_FROM(a, "'elif' must match an if-statement here") }
-    | a='else' ':' { RAISE_SYNTAX_ERROR_STARTING_FROM(a, "'else' must match a valid statement here") }
-
 invalid_with_stmt:
     | ['async'] 'with' ','.(expression ['as' star_target])+ NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
     | ['async'] 'with' '(' ','.(expressions ['as' star_target])+ ','? ')' NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }

Lib/test/test_syntax.py

@@ -1853,28 +1853,6 @@ Corner-cases that used to crash:
     Traceback (most recent call last):
     SyntaxError: positional patterns follow keyword patterns
 
-Non-matching 'elif'/'else' statements:
-
-    >>> if a == b:
-    ...     ...
-    ...     elif a == c:
-    Traceback (most recent call last):
-    SyntaxError: 'elif' must match an if-statement here
-
-    >>> if x == y:
-    ...     ...
-    ...     else:
-    Traceback (most recent call last):
-    SyntaxError: 'else' must match a valid statement here
-
-    >>> elif m == n:
-    Traceback (most recent call last):
-    SyntaxError: 'elif' must match an if-statement here
-
-    >>> else:
-    Traceback (most recent call last):
-    SyntaxError: 'else' must match a valid statement here
-
 Uses of the star operator which should fail:
 
 A[:*b]
@@ -2167,8 +2145,8 @@ class SyntaxTestCase(unittest.TestCase):
                      lineno=None, offset=None, end_lineno=None, end_offset=None):
         """Check that compiling code raises SyntaxError with errtext.
 
-        errtext is a regular expression that must be present in the
-        test of the exception raised. If subclass is specified, it
+        errtest is a regular expression that must be present in the
+        test of the exception raised.  If subclass is specified it
         is the expected subclass of SyntaxError (e.g. IndentationError).
         """
         try:
@@ -2192,22 +2170,6 @@ class SyntaxTestCase(unittest.TestCase):
         else:
             self.fail("compile() did not raise SyntaxError")
 
-    def _check_noerror(self, code,
-                       errtext="compile() raised unexpected SyntaxError",
-                       filename="<testcase>", mode="exec", subclass=None):
-        """Check that compiling code does not raise a SyntaxError.
-
-        errtext is the message passed to self.fail if there is
-        a SyntaxError. If the subclass parameter is specified,
-        it is the subclass of SyntaxError (e.g. IndentationError)
-        that the raised error is checked against.
-        """
-        try:
-            compile(code, filename, mode)
-        except SyntaxError as err:
-            if (not subclass) or isinstance(err, subclass):
-                self.fail(errtext)
-
     def test_expression_with_assignment(self):
         self._check_error(
             "print(end1 + end2 = ' ')",
@@ -2609,25 +2571,6 @@ while 1:
 """
         self._check_error(source, "too many statically nested blocks")
 
-    def test_syntax_error_non_matching_elif_else_statements(self):
-        # Check bpo-45759: 'elif' statements that doesn't match an
-        # if-statement or 'else' statements that doesn't match any
-        # valid else-able statement (e.g. 'while')
-        self._check_error(
-            "elif m == n:\n    ...",
-            "'elif' must match an if-statement here")
-        self._check_error(
-            "else:\n    ...",
-            "'else' must match a valid statement here")
-        self._check_noerror("if a == b:\n    ...\nelif a == c:\n    ...")
-        self._check_noerror("if x == y:\n    ...\nelse:\n    ...")
-        self._check_error(
-            "else = 123",
-            "invalid syntax")
-        self._check_error(
-            "elif 55 = 123",
-            "cannot assign to literal here")
-
     @support.cpython_only
     def test_error_on_parser_stack_overflow(self):
         source = "-" * 100000 + "4"

Misc/NEWS.d/next/Core and Builtins/2024-06-03-13-25-04.gh-issue-119724.EH1dkA.rst

@@ -0,0 +1,3 @@
+Reverted improvements to error messages for ``elif``/``else`` statements not
+matching any valid statements, which made in hard to locate the syntax
+errors inside those ``elif``/``else`` blocks.