fix panic: Overload evaluation: expected arity mismatch not found #4192

pyrefly/lib/alt/overload.rs

@@ -647,6 +647,7 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
             let mut arg_counts = closest_overload_signature.arg_counts();
             let nposargs = args.len();
             let nkwargs = keywords.len();
+            let missing_self_param = self_obj.is_some() && arg_counts.positional.max == Some(0);
             if self_obj.is_some() {
                 arg_counts.positional.min = arg_counts.positional.min.saturating_sub(1);
                 if let Some(max) = arg_counts.positional.max {
@@ -675,13 +676,19 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
                     None
                 }
             };
-            let arity_mismatch =
+            let arity_mismatch = if missing_self_param {
+                format!(
+                    "Expected 0 positional arguments, got {} (including implicit `self`)",
+                    nposargs + 1
+                )
+            } else {
                 check(nposargs + nkwargs, arg_counts.overall, "").unwrap_or_else(|| {
                     check(nposargs, arg_counts.positional, "positional ").unwrap_or_else(|| {
                         check(nkwargs, arg_counts.keyword, "keyword ")
                             .expect("Overload evaluation: expected arity mismatch not found")
                     })
-                });
+                })
+            };
             builder = builder.with_detail(arity_mismatch);
         } else {
             builder = builder.with_errors_as_details(closest_overload_call_errors);

pyrefly/lib/test/constructors.rs

@@ -703,6 +703,22 @@ assert_type(C(False), C[B])
     "#,
 );
 
+// Regression test for https://github.com/facebook/pyrefly/issues/4192
+testcase!(
+    test_overloaded_init_missing_self,
+    r#"
+from typing import overload
+
+class C:
+    @overload
+    def __init__(): ...  # E: Overloaded function must have an implementation
+    @overload
+    def __init__(): ...
+
+C()  # E: Expected 0 positional arguments, got 1 (including implicit `self`)
+    "#,
+);
+
 testcase!(
     test_init_bad_receiver_annotation,
     r#"