desolve y'=(x+y)^2

[parisse]

Indeed, you can also solve it by this kind of substitution. Maybe I can add another special case in desolve.
Your equation returns
(c_1*x^2+c_2*x+c_2)/(c_1*x+c_2)
with the latest Xcas. You can get your solution with c_2=1.
You are missing y=x with your general solution.