# ctt.3 — `KindMismatch` retire — Implementation Plan

> **Parent spec:** `docs/specs/2026-05-12-ct-tidy.md`
>
> **For agentic workers:** REQUIRED SUB-SKILL: use `skills/implement`
> to run this plan. Steps use `- [ ]` checkboxes for tracking.

**Goal:** Delete the `WorkspaceLoadError::KindMismatch` variant,
its helper `walk_kind_mismatch`, its dispatch site in
`validate_classdefs`, its Display arm in `crates/ail/src/main.rs`,
and the now-stale "dead-but-defensive" doc-comment on the test
that ratifies the canonical-form-rejection path. Plus two adjacent
textual-consistency edits surfaced during recon: the
`validate_classdefs` doc-comment ("Three diagnostics ..." → "Two")
and a stale `KindMismatch` mention in
`crates/ailang-check/src/lib.rs:1428`'s 22b.1-era narration.

**Architecture:** Pure mechanical deletion across two files in one
atomic edit pass. The variant deletion and the Display-arm deletion
form a lockstep producer/consumer pair — both must land before
`cargo build` is green. The existing green test
`class_param_in_applied_position_fires_canonical_form_rejection`
at `crates/ailang-core/src/workspace.rs:1888` stays unchanged; it
asserts `BareCrossModuleTypeRef`, not `KindMismatch`, so the
deletion does not affect its body.

**Tech Stack:** `ailang-core` (`workspace.rs`), `ail`
(`main.rs:workspace_error_to_diagnostic`), `ailang-check`
(`lib.rs` stale-comment reference).

**Files this plan creates or modifies:**

- Modify: `crates/ailang-core/src/workspace.rs:256-270` — delete
  `KindMismatch` variant + its doc + `#[error(...)]` attribute.
- Modify: `crates/ailang-core/src/workspace.rs:791-808` — drop
  `kind-mismatch` from the `validate_classdefs` doc-comment
  ("Three" → "Two") and delete the dispatch loop body.
- Modify: `crates/ailang-core/src/workspace.rs:844-867` — delete
  `walk_kind_mismatch` helper + its doc.
- Modify: `crates/ailang-core/src/workspace.rs:1879-1887` —
  delete the "dead-but-defensive" doc-comment block on
  `class_param_in_applied_position_fires_canonical_form_rejection`.
  The test name alone documents the remaining behaviour.
- Modify: `crates/ail/src/main.rs:1163-1182` — delete
  `W::KindMismatch { .. } => ...` arm in
  `workspace_error_to_diagnostic`.
- Modify: `crates/ailang-check/src/lib.rs:1427-1430` — drop the
  `KindMismatch` mention from the 22b.1-era doc-comment block
  (narrates which class-schema diagnostics 22b.1 deferred to 22b.2;
  post-ctt.3, `KindMismatch` is retired entirely).

**Design notes (settled pre-plan):**

- Stale-comment ref at `lib.rs:1428` is in scope. Reason: the block
  comment narrates iter 22b.1's deferred-to-22b.2 work and names
  three class-schema diagnostics including `KindMismatch`. After
  ctt.3 the diagnostic is retired; leaving the mention is
  archaeological noise that would catch a future `grep KindMismatch`
  unnecessarily and confuse a reader expecting the diagnostic to
  exist.
- `validate_classdefs` doc-comment at `workspace.rs:791-793` is in
  scope. Reason: the comment claims "Three diagnostics fire from
  here" but after the dispatch-loop deletion only two will. Adjacent
  textual consistency.
- "Dead-but-defensive" doc-comment at `workspace.rs:1879-1887` is
  deleted entirely (no successor). Reason: the test name
  `class_param_in_applied_position_fires_canonical_form_rejection`
  is self-documenting (it names the input shape, the output
  diagnostic-class, and the canonical-form path); a successor
  rationale doc would be redundant.

---

### Task 1: Atomic deletion pass

**Files:**
- Modify: `crates/ailang-core/src/workspace.rs` (4 deletion regions)
- Modify: `crates/ail/src/main.rs:1163-1182` (1 deletion region)
- Modify: `crates/ailang-check/src/lib.rs:1427-1430` (1 textual edit)

All edits land together. Between Step 1 and Step 6 the workspace
will not compile; this is the same atomic-pass shape ctt.2 used
(see ctt.2 plan's "Build must end green at Step 8"). `cargo build`
green at Step 7.

- [ ] **Step 1: Delete `WorkspaceLoadError::KindMismatch` variant**

Apply Edit to `crates/ailang-core/src/workspace.rs`:

`old_string`:
```
    /// Iter 22b.2: class-schema validation. The class parameter
    /// `f` (e.g. in `class C f { foo : f Int -> f Int }`) is kind
    /// `*` only — it may appear as a `Type::Var` in method
    /// signatures but never as a `Type::Con` head with arguments.
    /// A bare `f` used as a constructor name is a kind-mismatch.
    /// Code: `kind-mismatch`.
    #[error(
        "kind mismatch in class `{class}`: parameter `{param}` is used in applied position \
        inside method `{method}` (Decision 11 axis 5: class params are kind `*` only)"
    )]
    KindMismatch {
        class: String,
        param: String,
        method: String,
        defining_module: String,
    },

    /// Iter 22b.2: class-schema validation. A class's `superclass.type`
```

`new_string`:
```
    /// Iter 22b.2: class-schema validation. A class's `superclass.type`
```

Verifies: variant + its doc + its `#[error]` attribute are all
gone; the following sibling `InvalidSuperclassParam` immediately
follows. Surrounding doc preserves the next sibling intact.

If the exact `old_string` does not match (e.g. the doc-comment
wording shifted between recon and execution), the implementer
expands the `old_string` to a larger surrounding block until the
match is unambiguous.

- [ ] **Step 2: Adjust `validate_classdefs` doc-comment + delete dispatch loop**

Apply Edit to `crates/ailang-core/src/workspace.rs`:

`old_string`:
```
    /// Iter 22b.2: class-schema validation. Runs before `build_registry`.
    /// Three diagnostics fire from here: `kind-mismatch`,
    /// `invalid-superclass-param`, `constraint-references-unbound-type-var`.
```

`new_string`:
```
    /// Iter 22b.2: class-schema validation. Runs before `build_registry`.
    /// Two diagnostics fire from here: `invalid-superclass-param`,
    /// `constraint-references-unbound-type-var`. The `kind-mismatch`
    /// diagnostic was retired at ctt.3 — the malformed shape now
    /// fires `BareCrossModuleTypeRef` from the canonical-form
    /// validator before `validate_classdefs` runs.
```

Apply Edit to `crates/ailang-core/src/workspace.rs` (delete the
dispatch loop):

`old_string`:
```
            if let Def::Class(c) = def {
                for method in &c.methods {
                    walk_kind_mismatch(&method.ty, &c.param)
                        .map_err(|()| WorkspaceLoadError::KindMismatch {
                            class: c.name.clone(),
                            param: c.param.clone(),
                            method: method.name.clone(),
                            defining_module: mod_name.clone(),
                        })?;
                }
                if let Some(sc) = &c.superclass {
```

`new_string`:
```
            if let Def::Class(c) = def {
                if let Some(sc) = &c.superclass {
```

Verifies: the `Def::Class(c)` arm's body now begins directly with
the `if let Some(sc) = &c.superclass` check; the second
`for method in &c.methods { ... }` loop (constraint-validation pass)
at line ~819 stays untouched.

- [ ] **Step 3: Delete `walk_kind_mismatch` helper**

Apply Edit to `crates/ailang-core/src/workspace.rs`:

`old_string`:
```
/// Walks a `Type` looking for any `Type::Con { name == param, args
/// non-empty }`. The class param is kind `*`; appearing as a
/// constructor with arguments is a kind-mismatch.
fn walk_kind_mismatch(t: &Type, param: &str) -> Result<(), ()> {
    match t {
        Type::Con { name, args } => {
            if name == param && !args.is_empty() {
                return Err(());
            }
            for a in args {
                walk_kind_mismatch(a, param)?;
            }
            Ok(())
        }
        Type::Fn { params, ret, .. } => {
            for p in params {
                walk_kind_mismatch(p, param)?;
            }
            walk_kind_mismatch(ret, param)
        }
        Type::Forall { body, .. } => walk_kind_mismatch(body, param),
        Type::Var { .. } => Ok(()),
    }
}
```

`new_string`:
```
```

Verifies: the entire helper fn + its doc are gone; the next item
in the file (`is_primitive_type_name` or its doc) follows
immediately.

If a trailing blank line collapses or doubles, the Boss tidies
up at commit time — not a plan-step concern.

- [ ] **Step 4: Delete "dead-but-defensive" doc-comment on the canonical-form-rejection test**

Apply Edit to `crates/ailang-core/src/workspace.rs`:

`old_string`:
```
    /// ct.1: a class whose parameter `f` appears as a `Type::Con`
    /// name (the malformed-but-historically-test-fixture shape used
    /// to trigger `KindMismatch` pre-ct.1) is now caught earlier by
    /// the canonical-type-names validator: `f` is bare,
    /// non-primitive, and not a TypeDef in the owning module, so
    /// `BareCrossModuleTypeRef` fires before `validate_classdefs`
    /// gets a chance to run. The `KindMismatch` path stays in the
    /// codebase as dead-but-defensive code; a future tidy may
    /// retire it.
    #[test]
    fn class_param_in_applied_position_fires_canonical_form_rejection() {
```

`new_string`:
```
    #[test]
    fn class_param_in_applied_position_fires_canonical_form_rejection() {
```

Verifies: the 9-line doc-comment is gone; `#[test]` attribute
remains immediately above the fn declaration; the test body itself
(asserting `BareCrossModuleTypeRef`) is untouched.

- [ ] **Step 5: Delete `W::KindMismatch` Display arm in `main.rs`**

Apply Edit to `crates/ail/src/main.rs`:

`old_string`:
```
        W::KindMismatch {
            class,
            param,
            method,
            defining_module,
        } => Some((
            "kind-mismatch",
            format!(
                "kind mismatch in class `{class}`: parameter `{param}` is used in applied \
                 position inside method `{method}` (Decision 11 axis 5: class params are kind \
                 `*` only)"
            ),
            defining_module.clone(),
            Some(serde_json::json!({
                "class": class,
                "param": param,
                "method": method,
            })),
        ),
```

`new_string`:
```
```

Verifies: the `W::KindMismatch` match arm is gone; the surrounding
`match` over `W::*` variants stays exhaustive by construction
(Step 1's variant deletion makes the now-absent arm a non-issue
for compile-time exhaustiveness).

Note: the exact bracket / comma sequence of the deleted arm may
vary by a trailing comma or semicolon. If the `Edit` call fails
due to a mismatched terminator, the implementer expands the
`old_string` to include the preceding sibling-arm's closing
`),` and the following sibling-arm's opening identifier (e.g.
`W::InvalidSuperclassParam {`) to anchor unambiguously, then
adjusts `new_string` to preserve those anchors.

- [ ] **Step 6: Drop `KindMismatch` mention from `lib.rs:1428` archaeology comment**

Apply Edit to `crates/ailang-check/src/lib.rs`:

`old_string`:
```
        // Iter 22b.1: schema-only landing for class/instance defs.
        // Class-schema validation (KindMismatch, InvalidSuperclassParam,
        // ConstraintReferencesUnboundTypeVar) and instance-body
        // typechecking (with class-method substitution) land in 22b.2.
```

`new_string`:
```
        // Iter 22b.1: schema-only landing for class/instance defs.
        // Class-schema validation (InvalidSuperclassParam,
        // ConstraintReferencesUnboundTypeVar) and instance-body
        // typechecking (with class-method substitution) land in 22b.2.
        // ctt.3: KindMismatch retired — the malformed-class-param
        // shape now fires BareCrossModuleTypeRef from the canonical-
        // form validator instead.
```

Verifies: the `KindMismatch` token no longer appears in the comment;
a one-line successor sentence names where the diagnostic lives now.

- [ ] **Step 7: Build, confirm no compile errors**

Run: `cargo build --workspace`
Expected: clean build, no errors, no dead-code warnings (the
`match` on `W::*` in `main.rs:workspace_error_to_diagnostic`
stays exhaustive; the deleted variant is no longer in the source
of truth, so no orphan arm to flag).

If a compile error surfaces, it will most likely be:
- A test under `crates/` that references `KindMismatch` by name.
  `grep -rn "KindMismatch\|walk_kind_mismatch" crates/` should
  return zero hits after Step 6; if any hit remains, that site
  is the source of the compile error and needs the same
  treatment (deletion or rewording).
- A snapshot file that contains the `kind-mismatch` diagnostic
  string. Recon found no such snapshot, but a `find . -name
  '*.snap*' | xargs grep -l kind-mismatch` will surface one if
  present.

---

### Task 2: Workspace-wide test gate

**Files:** (verification only — no edits)

- [ ] **Step 1: Verify zero residual `KindMismatch` references**

Run: `grep -rn "KindMismatch\|walk_kind_mismatch" crates/ docs/`
Expected: zero hits in `crates/`. `docs/` may carry historical
references in journals or archived design docs — those are
archaeology and stay.

If `crates/` still has hits, the deletion was incomplete and
Task 1 reopens until clean.

- [ ] **Step 2: Run the canonical-form-rejection test, confirm GREEN**

Run: `cargo test -p ailang-core class_param_in_applied_position_fires_canonical_form_rejection`
Expected: PASS. The test was already green pre-ctt.3 — the
deletion does not touch its body, only its doc-comment.

- [ ] **Step 3: Run the entire workspace test suite**

Run: `cargo test --workspace`
Expected: all tests pass.

If a previously-green test goes red: most plausible cause is a
test that observed `W::KindMismatch` shape (either in JSON output
matching `"code": "kind-mismatch"`, or in human-mode output
matching the variant's `Display` text, or in a Rust-side pattern
match against the variant). Recon found no such test; if one
surfaces, inspect and either retire it (if it was pinning the
now-retired path) or migrate its assertion to
`BareCrossModuleTypeRef` (the canonical-form-rejection successor).

- [ ] **Step 4: Bench scripts deferred to audit**

ctt.3 makes zero codegen-relevant edits — the deleted code path
is workspace-load-time-only and is structurally unreachable
pre-ctt.3. No bench movement plausibly attributable; the
milestone-close audit at the end of the ct-tidy milestone
confirms.

---

## Acceptance criteria recap (from spec)

- `WorkspaceLoadError::KindMismatch`, `walk_kind_mismatch`, its
  dispatch site in `validate_classdefs`, and its `main.rs`
  Display arm are all deleted. ✓ Task 1 Steps 1, 2, 3, 5.
- The existing test
  `class_param_in_applied_position_fires_canonical_form_rejection`
  still passes, asserting `BareCrossModuleTypeRef`. ✓ Task 2
  Step 2.
- `cargo build --workspace` clean, no orphan match arms, no
  dead-code warnings. ✓ Task 1 Step 7.
- `cargo test --workspace` green. ✓ Task 2 Step 3.
- `grep KindMismatch crates/` returns no hits. ✓ Task 2 Step 1.
- Adjacent textual-consistency edits (validate_classdefs
  doc-comment "Three"→"Two", lib.rs:1428 archaeology comment)
  are folded in. ✓ Task 1 Steps 2 and 6.
