tests/check_new_syntax.py: check order of if statements (#6423)

tests/check_new_syntax.py

@@ -117,7 +117,18 @@ def check_new_syntax(tree: ast.AST, path: Path) -> list[str]:
                 self.old_syntax_finder().visit(node.returns)
             self.generic_visit(node)
 
+    class IfFinder(ast.NodeVisitor):
+        def visit_If(self, node: ast.If) -> None:
+            if isinstance(node.test, ast.Compare) and ast.unparse(node.test).startswith("sys.version_info < ") and node.orelse:
+                new_syntax = "if " + ast.unparse(node.test).replace("<", ">=", 1)
+                errors.append(
+                    f"{path}:{node.lineno}: When using if/else with sys.version_info, "
+                    f"put the code for new Python versions first, e.g. `{new_syntax}`"
+                )
+            self.generic_visit(node)
+
     AnnotationFinder().visit(tree)
+    IfFinder().visit(tree)
     return errors