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fix(fixpoint): matchfix termination check must descend into all call arguments #1144

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fix(fixpoint): matchfix termination check must descend into all call arguments #1144

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3 changes: 2 additions & 1 deletion src/ecHiInductive.ml
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Original file line number Diff line number Diff line change
Expand Up @@ -466,7 +466,8 @@ let trans_matchfix
match destr_app e with
| ({ e_node = Elocal x }, args) when x = opname -> begin
match List.nth_opt args pos with
| Some { e_node = Elocal a } when Sid.mem a pvars -> ()
| Some { e_node = Elocal a } when Sid.mem a pvars ->
List.iter check_body args

@lyonel2017 lyonel2017 Oct 4, 2026

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This check can be put outside the begin end block; this would be cleaner:

          begin
            match List.nth_opt args pos with
            | Some { e_node = Elocal a } when Sid.mem a pvars -> ()
            | _ -> fxerror loc env TT.FXE_SynCheckFailure
          end;
          List.iter check_body args

| _ -> fxerror loc env TT.FXE_SynCheckFailure
end
| _ -> EcTypes.e_iter check_body e
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15 changes: 15 additions & 0 deletions tests/matchfix-termination.ec
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Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
(* Regression for the matchfix termination-check hole.

`check_body` (ecHiInductive.ml) accepts a recursive call whose decreasing
argument is a valid structural subterm, but used to return without checking
the call's OTHER arguments -- so a non-decreasing recursive call hidden in a
sibling argument escaped. The op below defines `f (S Z) b = ! (f (S Z) b)`
(i.e. `f x = not (f x)`), which has no total-function solution and makes the
logic inconsistent. It MUST be rejected by the termination check. *)
require import AllCore.

type t = [ Z | S of t ].

fail op f (n : t) (b : bool) : bool =
with n = Z => b
with n = S k => f k (! (f n b)).
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