scopedLocalInsts.lean:12:6-12:39: error(lean.synthInstanceFailed): failed to synthesize instance of type class
  ToString A

Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command.
"A.mk 10 20"
scopedLocalInsts.lean:21:6-21:39: error(lean.synthInstanceFailed): failed to synthesize instance of type class
  ToString A

Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command.
"{ x := 10, y := 20 }"
scopedLocalInsts.lean:32:6-32:39: error(lean.synthInstanceFailed): failed to synthesize instance of type class
  ToString A

Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command.
"A.mk 10 20"