Fix being able to call arg-less #[new] with any args from Python

Fixes #2748

newsfragments/2749.fixed.md

@@ -0,0 +1,2 @@
+Fix a bug that allowed `#[new]` pymethods with no arguments to be called from
+Python with any argument list.

pyo3-macros-backend/src/params.rs

@@ -32,10 +32,6 @@ pub fn impl_arg_params(
     py: &syn::Ident,
     fastcall: bool,
 ) -> Result<(TokenStream, Vec<TokenStream>)> {
-    if spec.signature.arguments.is_empty() {
-        return Ok((TokenStream::new(), vec![]));
-    }
-
     let args_array = syn::Ident::new("output", Span::call_site());
 
     if !fastcall && is_forwarded_args(&spec.signature) {

tests/test_class_new.rs

@@ -2,6 +2,7 @@
 
 use pyo3::exceptions::PyValueError;
 use pyo3::prelude::*;
+use pyo3::types::IntoPyDict;
 
 #[pyclass]
 struct EmptyClassWithNew {}
@@ -23,6 +24,12 @@ fn empty_class_with_new() {
             .unwrap()
             .downcast::<PyCell<EmptyClassWithNew>>()
             .is_ok());
+
+        // Calling with arbitrary args or kwargs is not ok
+        assert!(typeobj.call(("some", "args"), None).is_err());
+        assert!(typeobj
+            .call((), Some([("some", "kwarg")].into_py_dict(py)))
+            .is_err());
     });
 }