/-!
In the past, when an implicit argument couldn't be synthesized, the name of the argument got lost during elaboration.
Now it is saved and added to the error message.

In this example, that is 'n'.
-/

def foo {n : Nat} := 2*n

/--
error: don't know how to synthesize implicit argument 'n'
  @foo ?_
context:
⊢ Nat
-/
#guard_msgs in
set_option pp.mvars false in
#eval foo