; RED for fieldtest finding B2 (raw-buf milestone, Gitea #7).
;
; Property: a `loop` whose result is `let`-bound, and whose body
; references one of its own loop binders on the non-recur exit, must
; BUILD — not only `check`. The let-lowering in codegen calls
; `synth_arg_type` on every let value to replay its AILang type; that
; type-replay walk must descend through `Term::Loop` with the loop's
; binders in scope, exactly as it already does for `Term::Let`.
;
; The fieldtest surfaced this with an owned `RawBuf` loop binder, but
; the trigger is element-type-independent: it is the `let`-bound loop
; with a binder reference reachable through the synth body-walk. A
; plain `Int` binder is the minimal autonomous reproduction.
;
; Pre-fix symptom: `ail check` ok, `ail build` ->
;   Error: module `loop_let_bound_binder_ref`: def `main`:
;     unknown variable: `b`
;
; Expected stdout: 3  (loop counts i 0,1,2 and returns binder b == i).
(module loop_let_bound_binder_ref
  (fn main
    (type (fn-type (params) (ret (con Unit)) (effects IO)))
    (params)
    (body
      (let r
        (loop (b (con Int) 0) (i (con Int) 0)
          (if (app ge i 3)
              b
              (recur (app + i 1) (app + i 1))))
        (app print r)))))