I have never looked at the trace of a decorated object before.  The 3.7 behavior treating the inner decorator line as the first line of the decorated function definition looks wrong to me.  I actually expected the line pointer to move down to the def line, analogously to the following, at least until after MAKE_FUNCTION, but moving to the beginning of the statement for the rest would seem proper.

>>> dis.dis("""a = f(
f(
f(
3)))""")
  1           0 LOAD_NAME                0 (f)

  2           2 LOAD_NAME                0 (f)

  3           4 LOAD_NAME                0 (f)

  4           6 LOAD_CONST               0 (3)
              8 CALL_FUNCTION            1
             10 CALL_FUNCTION            1
             12 CALL_FUNCTION            1
             14 STORE_NAME               1 (a)
             16 LOAD_CONST               1 (None)
             18 RETURN_VALUE