; Iter 18g tidy follow-up RED-test fixture: a let-binder whose
; value is a Term::Var referencing a non-Own fn-param must
; inherit the param's mode for codegen's drop-emission gates.
;
; The carve-out documented in 18d.4's Iter A fix: the gate
; checks `scrutinee_is_owned` against `current_param_modes`,
; but only fn-params are recorded there. A let-binder aliasing
; an Implicit-mode fn-param passes the gate (its lookup
; misses, default = owned), so the arm-close drop fires on
; memory the caller still references.
;
; Pattern: `pin_aliased` takes `t` as Implicit-mode (default,
; no annotation); aliases it via `(let a t ...)`; matches `a`.
; In the TNode arm the pattern-bindings `l` / `r` would be
; dec'd at arm close because `a` looks owned to the gate —
; corrupting the caller's tree.
;
; Pre-fix under --alloc=rc: refcount underflow at the second
; iteration of `loop`, where the recursive call into
; `pin_aliased(t)` re-loads cells the previous iteration
; freed.
; Post-fix: clean exit, output `0`. Stats are not the focus
; here (the caller's tree leaks under the Implicit-mode
; back-compat lane); the test asserts only correctness.

(module rc_let_alias_implicit_param

  (data Tree
    (ctor TLeaf)
    (ctor TNode (con Int) (con Tree) (con Tree)))

  (fn build
    (type
      (fn-type
        (params (con Int))
        (ret (own (con Tree)))))
    (params d)
    (body
      (if (app == d 0)
        (term-ctor Tree TLeaf)
        (term-ctor Tree TNode 1
          (app build (app - d 1))
          (app build (app - d 1))))))

  ; Pin via let-alias: the alias `a` is a Var-binding on the
  ; Implicit-mode (default) param `t`. Codegen's gate must
  ; recognise that `a` inherits `t`'s mode and skip the arm-
  ; close drop that would otherwise fragment the caller's
  ; tree.
  (fn pin_aliased
    (type
      (fn-type
        (params (con Tree))
        (ret (con Int))))
    (params t)
    (body
      (let a t
        (match a
          (case (pat-ctor TLeaf) 0)
          (case (pat-ctor TNode v l r) 1)))))

  (fn loop
    (type
      (fn-type
        (params (con Int) (con Tree))
        (ret (con Int))))
    (params n t)
    (body
      (if (app == n 0)
        0
        (let _v (app pin_aliased t)
          (app loop (app - n 1) t)))))

  (fn main
    (type (fn-type (params) (ret (con Unit)) (effects IO)))
    (params)
    (body
      (let t (app build 2)
        (do io/print_int (app loop 3 t))))))
