Match context pattern inside a tactic/tactic notation

Question

I find a pattern inside my goal through a tactic.

Why does this fail:

        Tactic Notation "my_context_match" uconstr(g) :=
          match goal with
          | |- context[g] => idtac
          end.

          my_context_match _.

While this succeeds?

        match goal with
          | |- context[_] => idtac
        end.

---

Is there any way to write a my_context_match, such that I can pass incomplete patterns (with _ on them) and see if anything inside my goal matches the patter?

Answer 1

Support for uconstr is very patchy. I've just reported #9321. Note that even this fails:

Goal True.
  let v := uconstr:(True) in
  lazymatch constr:(v) with
  | v => idtac
  end. (* Error: No matching clauses for match. *)

---

As suggested by @eponier in a comment, you can use open_constr instead of uconstr. However, this will leave unresolved evars. Here is a tactic that will work, and will not leave unresolved evars:

Tactic Notation "my_context_match" uconstr(g) :=
  (* [match] does not support [uconstr], cf COQBUG(https://github.com/coq/coq/issues/9321),
     so we use [open_constr] *)
  let g := open_constr:(g) in
  (* turning [g] into an [open_constr] creates new evars, so we must
     eventually unify them with the goal *)
  let G := match goal with |- ?G => G end in
  (* We now search for [g] in the goal, and then replace the matching
     subterm with the [open_constr] [g], so that we can unify the
     result with the goal [G] to resolve the new evars we created *)
  match G with
  | context cG[g]
    => let G' := context cG[g] in
       unify G G'
  end.

Goal True /\ True.
  my_context_match _.
  my_context_match (_ /\ _).
  Fail my_context_match (_ \/ _).
  my_context_match True.
  exact (conj I I).
Qed.