Add test.

tests/decidable.ref

+The command has indeed failed with message:
+No applicable tactic.

tests/decidable.v

+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_option_guard; [|done].
+  Fail done. (* Would succeed if the instance backing [x ∈ xs] is infered as
+  [False]. *)
+Abort.