#@ load("@ytt:overlay", "overlay")
#@ load("@ytt:assert", "assert")

#@ def test1_left():
key1: val1
#@ end

#@ def test1_right():
#@overlay/assert via=lambda a,b: 1
key1: val2
#@ end

test1: #@ overlay.apply(test1_left(), test1_right())

+++

ERR: 
- overlay.apply: Map item (key 'key1') on line stdin:10: Expected via invocation to return NoneType, Bool or Tuple(Bool,String), but returned neither of those
    in <toplevel>
      stdin:13 | test1: #@ overlay.apply(test1_left(), test1_right())