iter 22b.2.10: fix — positive green-path test, drop dead let _ = ws

Adds the missing positive counterpart to the no-instance test:
`show 42` with `instance Show Int` PRESENT must typecheck green.
This guards against registry-keying or threading bugs that would
otherwise sneak past the negative-only test (an empty registry at
typecheck time, or a key shape mismatch, would silently fire
no-instance even when the instance exists).

Also drops the dead `let _ = ws;` and its stale comment in
`check_in_workspace` — the line above already consumes
`ws.registry`, so the explicit drop is redundant. The remaining
comment is folded into the registry-threading comment so the
"registry is the only thing threaded" rationale is preserved.

crates/ail/tests/typeclass_22b2.rs

@@ -235,3 +235,26 @@ fn fn_calling_class_method_at_type_without_instance_fires_no_instance() {
         diagnostics.iter().map(|d| &d.code).collect::<Vec<_>>()
     );
 }
+
+/// Property protected: the positive counterpart to `no-instance` — when a
+/// monomorphic fn calls a class method at a fully-concrete type AND a
+/// matching `instance C T` exists in the workspace, the call typechecks
+/// green. The concrete-residual path must look up `(class, type_hash(T))`
+/// in `env.workspace_registry` and find a hit, so no diagnostic fires.
+/// This guards against registry-keying or threading bugs that would
+/// otherwise sneak past the negative-only test: a registry that is empty
+/// at typecheck time, or keyed under the wrong shape, would silently fire
+/// `no-instance` even when the instance is present. Fixture: `main` calls
+/// `show 42` with `instance Show Int` declared in the same workspace.
+#[test]
+fn fn_calling_class_method_at_type_with_instance_typechecks_green() {
+    let entry = examples_dir().join("test_22b2_instance_present.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 — instance Show Int is present, registry lookup must hit; got: {:?}",
+        diagnostics
+    );
+}

crates/ailang-check/src/lib.rs

@@ -1177,12 +1177,11 @@ fn check_in_workspace(
     env.class_superclasses = class_superclasses;
     // Iter 22b.2 (Task 10): thread the workspace-load-time instance
     // registry into the env so `check_fn`'s concrete-residual path can
-    // look up `(class, type_hash)` keys for the no-instance check.
+    // look up `(class, type_hash)` keys for the no-instance check. The
+    // registry is the only field of `ws` consumed at typecheck time;
+    // cross-module name resolution uses `module_globals` / `module_types`
+    // built earlier in this function.
     env.workspace_registry = ws.registry.clone();
-    // Workspace isn't otherwise needed in the env; cross-module lookup
-    // uses only `module_globals` / `module_types`. The ws reference is
-    // preserved in case cross-module ADTs are added later.
-    let _ = ws;
 
     for def in &m.defs {
         if let Err(e) = check_def(def, &env) {

examples/test_22b2_instance_present.ail.json

@@ -0,0 +1,40 @@
+{
+  "schema": "ailang/v0",
+  "name": "test_22b2_instance_present",
+  "imports": [],
+  "defs": [
+    {
+      "kind": "class", "name": "Show", "param": "a",
+      "methods": [{
+        "name": "show",
+        "type": {
+          "k": "fn", "params": [{ "k": "var", "name": "a" }],
+          "ret": { "k": "con", "name": "Str" }, "effects": []
+        }
+      }]
+    },
+    {
+      "kind": "instance",
+      "class": "Show",
+      "type": { "k": "con", "name": "Int" },
+      "methods": [{
+        "name": "show",
+        "body": { "t": "lit", "lit": { "kind": "str", "value": "n" } }
+      }]
+    },
+    {
+      "kind": "fn",
+      "name": "main",
+      "type": {
+        "k": "fn", "params": [], "ret": { "k": "con", "name": "Str" },
+        "effects": []
+      },
+      "params": [],
+      "body": {
+        "t": "app",
+        "fn": { "t": "var", "name": "show" },
+        "args": [{ "t": "lit", "lit": { "kind": "int", "value": 42 } }]
+      }
+    }
+  ]
+}