; Fieldtest mut-local #5 — deliberate probe of the seal-by-construction
; promise: try to lift a mut-var into a lambda closure.
;
; Spec §"Out of scope": "Lambda capture of a mut-var. A lambda body
; whose free vars include a mut-var of an enclosing Term::Mut is
; rejected at typecheck with CheckError::MutVarCapturedByLambda."
;
; This file is EXPECTED TO FAIL at `ail check` with the
; `mut-var-captured-by-lambda` diagnostic. The purpose is to probe:
;   - that the diagnostic actually fires
;   - that its rendered text is actionable
;   - that it points at the lambda site, not somewhere else
;
; The shape: a mut block declares `count`, builds a closure that would
; close over `count`, and tries to return the closure. An LLM-author
; might write this naïvely thinking "I just need a small callback that
; updates the running count" — exactly the shape the seal forbids.

(module mut-local_5_lambda_capture_probe

  (fn make_bumper
    (type
      (fn-type
        (params (con Int))
        (ret (fn-type (params (con Int)) (ret (con Int))))))
    (params seed)
    (body
      (mut
        (var count (con Int) 0)
        (assign count seed)
        (lam (params (typed n (con Int)))
             (ret (con Int))
             (body (app + n count))))))

  (fn main
    (type (fn-type (params) (ret (con Unit)) (effects IO)))
    (params)
    (body
      (app print (app (app make_bumper 10) 5)))))