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

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

#@ def test1_right():
#@overlay/assert
key1: val2
#@ end

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

+++

ERR: 
- overlay.apply: Map item (key 'key1') on line stdin:9: Expected objects to equal, but did not: Expected leaf values to match string string
    in <toplevel>
      stdin:12 | test1: #@ overlay.apply(test1_left(), test1_right())