iter 22b.2.10: no-instance per-fn diagnostic

crates/ail/tests/typeclass_22b2.rs

@@ -212,3 +212,26 @@ fn fn_with_superclass_constraint_calling_subclass_method_fires_missing_constrain
         diagnostics.iter().map(|d| &d.code).collect::<Vec<_>>()
     );
 }
+
+/// Property protected: a (monomorphic) fn that calls a class method at a
+/// fully-concrete type for which no `instance C T` exists in the workspace
+/// MUST receive a `no-instance` diagnostic. This is the dual of
+/// `missing-constraint`: when the residual class constraint resolves to a
+/// concrete type, the fn cannot push the obligation to a caller via a
+/// `Forall.constraints` annotation — it can only be discharged by an
+/// existing instance. Without this check, monomorphisation in 22b.3 would
+/// have nothing to specialise to. Fixture: `main` calls `show 42` (so the
+/// residual is `(Show, Int)`) without any `instance Show Int` declared in
+/// the workspace.
+#[test]
+fn fn_calling_class_method_at_type_without_instance_fires_no_instance() {
+    let entry = examples_dir().join("test_22b2_no_instance.ail.json");
+    let ws = ailang_core::workspace::load_workspace(&entry)
+        .expect("workspace loads");
+    let diagnostics = ailang_check::check_workspace(&ws);
+    assert!(
+        diagnostics.iter().any(|d| d.code == "no-instance"),
+        "expected no-instance; got: {:?}",
+        diagnostics.iter().map(|d| &d.code).collect::<Vec<_>>()
+    );
+}