# Iter 23.4-prep — Checker Prerequisites for Prelude Free Fns — Implementation Plan

> **Parent spec:** `docs/specs/2026-05-10-23-eq-ord-prelude.md`
>
> **Plan provenance:** This iter does not have a spec section of its
> own. It closes two checker gaps discovered mid-iter when iter 23.4
> BLOCKED on Task 1. See the BLOCKED journal at
> `git show iter/23.4:docs/journals/2026-05-11-iter-23.4.md`.
>
> **For agentic workers:** REQUIRED SUB-SKILL: use `skills/implement`
> to run this plan. Steps use `- [ ]` checkboxes for tracking.

**Goal:** Close two localised checker gaps so iter 23.4's prelude
free fns become reachable from user code: (Gap 1) extend bare-name
resolution to fall through to free fns in implicitly-imported
modules; (Gap 2) register `Def::Class` method types in
`linearity::check_module`'s globals so class-method `param_modes`
become visible to the borrow walk.

**Architecture:**

Gap 2 is purely intra-module: `ne` and `eq` both live in the prelude
module, so `linearity::check_module(prelude)` is the only call that
needs to see class-method modes. Fix iterates `class_def.methods`
in the build loop and inserts `(method_name, strip_forall(ty))` into
`globals`, mirroring the existing `Def::Fn(f)` arm two lines above.

Gap 1 is per-consumer-module workspace lookup: the `Term::Var { name }`
fall-through ladder gains one new arm between `env.class_methods`
and the dot-qualified arm. New arm iterates `env.imports`
(per-current-module implicit-import alias map) and consults
`env.module_globals[<mod>].fns.get(name)`. Single-match wins;
zero-match and multi-match both fall through (the latter is
unreachable today — prelude is the only implicit import — and is
covered by a one-line code comment naming the prelude-singleton
invariant). No new `Env` field; existing `env.imports` and
`env.module_globals` carry everything needed.

**Tech Stack:** `ailang-check` crate (`linearity.rs` for Gap 2,
`lib.rs` for Gap 1). No changes to `ailang-core`, `ailang-codegen`,
or any on-disk fixture (including the prelude).

**Out of scope:**
- The five prelude free fns themselves (`ne`/`lt`/`le`/`gt`/`ge`).
  They land in iter 23.4 unchanged from the existing
  `docs/plans/2026-05-11-iter-23.4.md`.
- Any modification to `examples/prelude.ail.json`.
- Any new diagnostic for the hypothetical multi-implicit-import
  ambiguity case.
- Extension of bare-name lookup to explicit imports (today's behaviour
  — explicit imports stay qualified-only — is preserved).

---

## Files this plan creates or modifies

- **Modify:** `crates/ailang-check/src/linearity.rs:212` — replace
  the empty `Def::Class(_) | Def::Instance(_) => {}` arm with one
  that iterates `class_def.methods` and registers each
  `(method_name, strip_forall(method_ty))` into the `globals`
  HashMap. `Def::Instance` stays a no-op (instance method bodies
  are walked separately as Def::Fn after monomorphisation; per the
  recon, the spec-23.4 false-positive flows from the class-method
  *type declaration*, not from the instance body).
- **Modify:** `crates/ailang-check/src/linearity.rs` test module —
  add `class_method_borrow_call_does_not_fire_consume_while_borrowed`
  unit test inside the existing `#[cfg(test)] mod tests` block.
- **Modify:** `crates/ailang-check/src/lib.rs` around line 1780 —
  extend the bare-name lookup ladder in the `Term::Var { name }` arm
  inside `synth` (signature at line 1745). Insert one new
  `else if` between the existing `env.class_methods.get(name)`
  branch and the dot-qualified arm. New branch iterates
  `env.imports.values()` (each value is the actual module name an
  implicit alias resolves to) and consults
  `env.module_globals[<mod>].fns.get(name)`. Single-match returns
  the substituted type. Zero or multi match falls through.
- **Modify:** `crates/ailang-check/src/lib.rs` test module (or
  similar pre-existing test location near the `Term::Var` arm) —
  add `bare_name_resolves_through_implicit_import_to_free_fn` test
  that constructs an in-memory `Workspace` with a synthetic prelude
  carrying a test-only free fn `dbl : Int -> Int` and a consumer
  module calling bare `dbl 5`. RED before the fix
  (`unknown-ident: dbl`), GREEN after.

---

## Task 1: Gap 2 — `linearity::check_module` registers class-method types

**Files:**
- Modify: `crates/ailang-check/src/linearity.rs:208–214` — replace
  the empty `Def::Class(_) | Def::Instance(_) => {}` arm.
- Modify: `crates/ailang-check/src/linearity.rs` test module — add
  one new unit test.

- [ ] **Step 1: Write the failing test inside the existing test module.**

Locate the `#[cfg(test)] mod tests { ... }` block in `linearity.rs`.
After the most recent existing test, append:

```rust
#[test]
fn class_method_borrow_call_does_not_fire_consume_while_borrowed() {
    // Construct a module with one `Def::Class` carrying a borrow-borrow
    // method, and one `Def::Fn` whose body forwards its borrow-mode
    // params into a call to that class method. The pre-fix behaviour
    // is `consume-while-borrowed` false positives on both params.
    use crate::ast::{
        ClassDef, ClassMethod, Def, FnDef, Module, ParamMode, Term, Type,
    };

    let class_eq = ClassDef {
        name: "TestEq".to_string(),
        param: "a".to_string(),
        superclass: None,
        methods: vec![ClassMethod {
            name: "teq".to_string(),
            ty: Type::Fn {
                params: vec![
                    Type::Var { name: "a".into() },
                    Type::Var { name: "a".into() },
                ],
                param_modes: vec![ParamMode::Borrow, ParamMode::Borrow],
                ret: Box::new(Type::Con { name: "Bool".into() }),
                ret_mode: ParamMode::Implicit,
                effects: vec![],
            },
            default: None,
        }],
        doc: None,
    };

    let ne = FnDef {
        name: "tne".to_string(),
        ty: Type::Fn {
            params: vec![
                Type::Var { name: "a".into() },
                Type::Var { name: "a".into() },
            ],
            param_modes: vec![ParamMode::Borrow, ParamMode::Borrow],
            ret: Type::Con { name: "Bool".into() }.into(),
            ret_mode: ParamMode::Implicit,
            effects: vec![],
        },
        params: vec!["x".into(), "y".into()],
        body: Term::App {
            fn_: Box::new(Term::Var { name: "teq".into() }),
            args: vec![
                Term::Var { name: "x".into() },
                Term::Var { name: "y".into() },
            ],
        },
        doc: None,
    };

    let module = Module {
        name: "m".into(),
        imports: vec![],
        defs: vec![Def::Class(class_eq), Def::Fn(ne)],
    };

    let diags = check_module(&module);
    assert!(
        diags.is_empty(),
        "expected no diagnostics but got {:#?}",
        diags
    );
}
```

If the exact AST constructor field names diverge from this snippet
(e.g. `fn_` vs `f`, `Box` placement on `ret`), use the existing
test in the file as the canonical shape — same enum variants, same
field ordering. The implementer surfaces NEEDS_CONTEXT only if the
field-name diagnostic is non-trivial; field-rename adjustments are
in-scope for this step.

- [ ] **Step 2: Run the test — expect RED.**

Run: `cargo test --workspace -p ailang-check class_method_borrow_call_does_not_fire_consume_while_borrowed`

Expected: FAIL. The test should panic on the `assert!` because two
`consume-while-borrowed` diagnostics fire — one for `x`, one for `y`
— from the synthesised `App(teq, [x, y])` since `callee_arg_modes`
fails to find `teq` in `globals` and defaults to `Consume`.

- [ ] **Step 3: Apply the fix.**

Replace `crates/ailang-check/src/linearity.rs:208–214`:

```rust
            // Iter 22b.1: class/instance defs are skipped here. Once
            // 22b.3 monomorphisation materialises class methods as
            // ordinary `FnDef`s, the lift's globals map will pick
            // them up automatically.
            Def::Class(_) | Def::Instance(_) => {}
```

with:

```rust
            // Iter 23.4-prep: class methods register their types into
            // the globals map so `callee_arg_modes` can see their
            // `param_modes`. Without this, a borrow-mode fn that
            // forwards its borrow params into a class-method call
            // fires `consume-while-borrowed` false positives.
            // Instance defs stay a no-op: instance method bodies are
            // walked separately via `Def::Fn` after monomorphisation.
            Def::Class(c) => {
                for m in &c.methods {
                    globals.insert(m.name.clone(), strip_forall(&m.ty).clone());
                }
            }
            Def::Instance(_) => {}
```

- [ ] **Step 4: Run the test — expect GREEN.**

Run: `cargo test --workspace -p ailang-check class_method_borrow_call_does_not_fire_consume_while_borrowed`

Expected: PASS.

- [ ] **Step 5: Full workspace sanity.**

Run: `cargo test --workspace`

Expected: PASS. No regression in any other linearity or check test.
If anything fires, root-cause; the most likely failure mode is a
test that relied on the old skip behaviour (we have no such test
on record — recon found none).

- [ ] **Step 6: Commit.**

```bash
git add crates/ailang-check/src/linearity.rs
git commit -m "iter 23.4-prep.1: linearity — register Def::Class method types in globals"
```

---

## Task 2: Gap 1 — bare-name resolution falls through to implicitly-imported free fns

**Files:**
- Modify: `crates/ailang-check/src/lib.rs` around line 1780 — insert
  one new branch into the `Term::Var { name }` lookup ladder inside
  `synth`.
- Modify: `crates/ailang-check/src/lib.rs` test module — add the
  bare-name test.

- [ ] **Step 1: Write the failing integration test.**

Locate the existing test module in `crates/ailang-check/src/lib.rs`
(near the bottom of the file — find `#[cfg(test)] mod tests`).
Append:

```rust
#[test]
fn bare_name_resolves_through_implicit_import_to_free_fn() {
    // Construct a workspace whose prelude carries a test-only free
    // fn `dbl : Int -> Int`, and a consumer module that calls bare
    // `dbl 5`. The pre-fix behaviour is `unknown-ident: dbl`,
    // because the bare-name lookup ladder consults the consumer's
    // local globals + workspace-flat class_methods but never the
    // free fns of implicitly-imported modules.
    use crate::ast::{Def, FnDef, Lit, Module, ParamMode, Term, Type};
    use crate::workspace::Workspace;

    let prelude = Module {
        name: "prelude".into(),
        imports: vec![],
        defs: vec![Def::Fn(FnDef {
            name: "dbl".into(),
            ty: Type::Fn {
                params: vec![Type::Con { name: "Int".into() }],
                param_modes: vec![ParamMode::Implicit],
                ret: Type::Con { name: "Int".into() }.into(),
                ret_mode: ParamMode::Implicit,
                effects: vec![],
            },
            params: vec!["n".into()],
            body: Term::Var { name: "n".into() },
            doc: None,
        })],
    };

    let consumer = Module {
        name: "m".into(),
        imports: vec![],
        defs: vec![Def::Fn(FnDef {
            name: "main".into(),
            ty: Type::Fn {
                params: vec![],
                param_modes: vec![],
                ret: Type::Con { name: "Int".into() }.into(),
                ret_mode: ParamMode::Implicit,
                effects: vec![],
            },
            params: vec![],
            body: Term::App {
                fn_: Box::new(Term::Var { name: "dbl".into() }),
                args: vec![Term::Lit {
                    lit: Lit::Int { value: 5 },
                }],
            },
            doc: None,
        })],
    };

    let ws = Workspace {
        entry: "m".into(),
        modules: vec![prelude, consumer].into_iter()
            .map(|m| (m.name.clone(), m))
            .collect(),
    };

    let mg = build_module_globals(&ws).expect("build_module_globals");
    let errs = check_in_workspace(&ws.modules["m"], &ws, &mg);
    assert!(
        errs.is_empty(),
        "expected no errors but got {:#?}",
        errs
    );
}
```

If a helper for synthetic-Workspace construction already exists
(e.g. `single_module_with_type_con` at `workspace.rs:2095+` per the
recon's cross-reference), prefer that helper's idiom. The exact
field names follow whatever the existing tests use.

- [ ] **Step 2: Run the test — expect RED.**

Run: `cargo test --workspace -p ailang-check bare_name_resolves_through_implicit_import_to_free_fn`

Expected: FAIL with `unknown-ident: dbl` (or the project's
equivalent — the bare-name lookup falls through every existing arm
and terminates at the `UnknownIdent` error at line 1842).

- [ ] **Step 3: Apply the fix to the `Term::Var { name }` lookup ladder.**

In `crates/ailang-check/src/lib.rs:1745+` (function `synth`),
locate the `Term::Var { name }` arm. After the existing
`env.class_methods.get(name)` branch (around line 1780) and before
the dot-qualified arm (around line 1819), insert:

```rust
            // Iter 23.4-prep: bare-name fall-through to free fns of
            // implicitly-imported modules. Iterates the current
            // module's implicit-import aliases (today: only `prelude`
            // — see lib.rs:1163-1180). Single-match wins; zero-match
            // and multi-match fall through (multi-match is
            // unreachable as long as `prelude` is the only implicit
            // import; if a second implicit import lands, revisit).
            } else if let Some((mod_name, fn_ty)) = env.imports
                .values()
                .filter_map(|mod_name| {
                    env.module_globals
                        .get(mod_name)
                        .and_then(|mg| mg.get(name).map(|ty| (mod_name.clone(), ty.clone())))
                })
                .next()
            {
                // Hash-mark the resolved owner so downstream passes
                // (codegen mangling, mono) can compute the cross-
                // module symbol. Same treatment the dot-qualified
                // arm gives below.
                let _ = mod_name; // owner reachable via the workspace; mangling consults
                                  // the FnDef's home module directly during codegen.
                let ty = substitute_freshly(&fn_ty);
                Ok((Term::Var { name: name.clone() }, ty))
```

Notes:
1. `env.imports.values()` yields the per-current-module list of
   resolved module names (per recon: implicit-prelude is wired at
   1271-1290 alongside the workspace-flat path).
2. The `substitute_freshly` (or equivalent type-instantiation helper)
   is the same one the existing globals branch uses around line 1773
   — match its name verbatim from the existing code.
3. The owner-module name is informational here; codegen mangles
   cross-module references via the FnDef's owning module field at
   monomorphisation time. The placeholder `let _ = mod_name;` makes
   the variable usage explicit; remove it once the implementer
   confirms (via existing class-method-resolution patterns) that no
   additional bookkeeping is needed at the `Term::Var` site.
4. The `.next()` short-circuits at the first match. This implements
   the single-match-wins rule. If two implicit imports both define
   the same name, the first iterated wins — recon flagged this as
   acceptable today (prelude singleton); the code comment names the
   future revisit point.

If the existing class-method branch at line 1780 uses a different
substitution helper or returns a more elaborate result shape (e.g.
inserts the resolved class instance), mirror that shape verbatim
for free fns — recon noted free fns have *less* than class methods
(no instance disambiguation needed), so any class-method-only
bookkeeping (e.g. instance candidate lists) does not apply to the
new branch.

- [ ] **Step 4: Run the test — expect GREEN.**

Run: `cargo test --workspace -p ailang-check bare_name_resolves_through_implicit_import_to_free_fn`

Expected: PASS.

- [ ] **Step 5: Full workspace sanity.**

Run: `cargo test --workspace`

Expected: PASS. The most plausible regression vectors are (a) a test
that relied on `unknown-ident` firing for a name that today lives
in a non-prelude module reachable via implicit-import wiring (recon
says no such test exists — prelude is the only implicit import), or
(b) a name-shadowing case where a local variable and a prelude free
fn share a name (the existing `locals → globals → class_methods`
ladder already shields locals; the new branch sits *after*
class_methods so locals still win).

- [ ] **Step 6: Commit.**

```bash
git add crates/ailang-check/src/lib.rs
git commit -m "iter 23.4-prep.2: check — bare-name fall-through to implicit-imported free fns"
```

---

## Plan self-review

1. **Coverage:** Two tasks, one per gap, both with RED-first unit /
   integration tests that close the failure mode the iter-23.4
   BLOCKED journal documented. ✓
2. **Placeholder scan:** No "TBD", "TODO", "implement later", or
   "similar to Task N". Task 2 Step 3 contains a deliberate
   "if the existing class-method branch ... mirror that shape
   verbatim" — this is *guidance for an implementer-side ambiguity
   that cannot be resolved without reading the unquoted surrounding
   code*, not a placeholder. The plan-recon snippet quoted enough
   to make the fix concrete; the mirror-clause covers a single
   localised judgment call (instance-list shape) that the implementer
   resolves by reading the same function block they're editing. ✓
3. **Type consistency:** `dbl`, `tne`/`teq` (synthetic names in test
   modules, isolated from prelude), `Workspace`, `Module`,
   `ClassDef`, `ClassMethod`, `FnDef`, `Term::Var`, `Term::App`,
   `Type::Fn`, `ParamMode::Borrow/Implicit`, `build_module_globals`,
   `check_in_workspace`, `check_module`, `strip_forall`,
   `callee_arg_modes`, `env.class_methods`, `env.module_globals`,
   `env.imports`. All referenced consistently across both tasks
   and matched to the recon's line anchors. ✓
4. **Step granularity:** Each step is one action plus a single
   `cargo test` invocation. Step 5 in each task adds a full-suite
   sanity (`cargo test --workspace`) — fast enough on this corpus
   to keep the step inside the 2-5 minute window. ✓
