iter 22b.2.9: missing-constraint per-fn diagnostic

crates/ail/tests/typeclass_22b2.rs

@@ -127,3 +127,45 @@ fn class_method_entry_carries_full_metadata() {
         ),
     }
 }
+
+/// Property protected: a polymorphic FnDef that calls a class method
+/// (e.g. `show x` where `x: a`) but does not declare the matching class
+/// constraint in its `Forall.constraints` MUST receive a
+/// `missing-constraint` diagnostic. This is the principal soundness
+/// guarantee of class-method calls inside polymorphic functions: every
+/// dispatch point at a polymorphic type must be backed by an in-scope
+/// constraint that the caller will eventually have to satisfy. Without
+/// this check, monomorphisation in 22b.3 would have no way to know
+/// which dictionary to thread.
+#[test]
+fn fn_calling_class_method_without_declared_constraint_fires_missing_constraint() {
+    let entry = examples_dir().join("test_22b2_missing_constraint.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 == "missing-constraint"),
+        "expected missing-constraint; got: {:?}",
+        diagnostics.iter().map(|d| &d.code).collect::<Vec<_>>()
+    );
+}
+
+/// Property protected: when a polymorphic FnDef declares the matching
+/// class constraint in its `Forall.constraints`, the call to the class
+/// method typechecks clean — the residual constraint produced at the
+/// call site is satisfied by the in-scope declared constraint, and no
+/// diagnostic fires. This is the positive counterpart to the
+/// missing-constraint check: declaring `Show a =>` makes `show x`
+/// well-formed inside the body.
+#[test]
+fn fn_with_correct_constraint_typechecks_green() {
+    let entry = examples_dir().join("test_22b2_constraint_declared.ail.json");
+    let ws = ailang_core::workspace::load_workspace(&entry)
+        .expect("workspace loads");
+    let diagnostics = ailang_check::check_workspace(&ws);
+    assert!(
+        diagnostics.is_empty(),
+        "expected no diagnostics; got: {:?}",
+        diagnostics
+    );
+}