import Lean


open Lean
open Lean.Elab
open Lean.Elab.Term

def getCtors (c : Name) : TermElabM (List Name) := do
let env ← getEnv;
(match env.find? c with
| some (ConstantInfo.inductInfo val) =>
  pure val.ctors
| _ => pure [])

def elabAnonCtor (args : Array (TSyntax `term)) (τ : Expr) : TermElabM Expr :=
  match τ.getAppFn with
  | Expr.const C _ => do
    let ctors ← getCtors C;
    (match ctors with
    | [c] => do
      let stx ← `($(Lean.mkIdent c) $args*);
      elabTerm stx τ
-- error handling
    | _ => unreachable!)
  | _ => unreachable!

elab "foo⟨" args:term,* "⟩" : term <= τ => do
  elabAnonCtor args τ

example : Nat × Nat := foo⟨1, 2⟩