gh-158320: Reject optional groups with a var-positional parameter in Argument Clinic (GH-158323)

Such a combination is ambiguous: the number of arguments does not tell
whether the group is passed or all arguments belong to the var-positional
parameter.

Author: serhiy-storchaka

Lib/test/test_clinic.py

@@ -2106,6 +2106,28 @@ def test_disallowed_grouping__no_matching_bracket(self):
         err = "Function 'empty_group' has a ']' without a matching '['"
         self.expect_failure(block, err)
 
+    def test_disallowed_grouping__varpos(self):
+        err = "cannot use optional groups with a var-positional parameter"
+        block = """
+            module foo
+            foo.bar
+                [
+                a: int
+                ]
+                *args: tuple
+        """
+        self.expect_failure(block, err, lineno=5)
+        block = """
+            module foo
+            foo.bar
+                a: int
+                *args: tuple
+                [
+                b: int
+                ]
+        """
+        self.expect_failure(block, err, lineno=4)
+
     def test_disallowed_grouping__must_be_position_only(self):
         dataset = ("""
             with_kwds

Tools/clinic/libclinic/dsl_parser.py

@@ -1161,6 +1161,12 @@ def bad_node(self, node: ast.AST) -> None:
 
         kind: inspect._ParameterKind
         if is_vararg:
+            if any(p.group for p in self.function.parameters.values()):
+                # With "foo([a, b], *args)" the number of arguments does not
+                # tell whether the group is passed or all arguments belong
+                # to the var-positional parameter.
+                fail(f"Function {self.function.name!r} cannot use optional "
+                     f"groups with a var-positional parameter.")
             kind = inspect.Parameter.VAR_POSITIONAL
         elif is_var_keyword:
             kind = inspect.Parameter.VAR_KEYWORD
@@ -1303,6 +1309,9 @@ def parse_star(self, function: Function, version: VersionTuple | None) -> None:
 
     def parse_opening_square_bracket(self, function: Function) -> None:
         """Parse opening parameter group symbol '['."""
+        if any(p.is_vararg() for p in function.parameters.values()):
+            fail(f"Function {function.name!r} cannot use optional groups "
+                 f"with a var-positional parameter.")
         # A group can only be nested in a group which does not contain
         # parameters yet, but two groups on the same nesting level can
         # follow each other.