File: decidable.v

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From stdpp Require Import list.

(** Test that Coq does not infer [x ∈ xs] as [False] by eagerly using
[False_dec] on a goal with unresolved type class instances. *)
Example issue_165 (x : nat) :
  ¬ ∃ xs : list nat, (guard (x ∈ xs);; Some x) ≠ None.
Proof.
  intros [xs Hxs]. case_guard; [|done].
  Fail done. (* Would succeed if the instance backing [x ∈ xs] is inferred as
  [False]. *)
Abort.