How can I define a boundary of a variable by another variable?

Hello,

I'm trying to graph a function; but one of the variables' boundaries is defined by another variable.

For instance: ezsurf(x^2 + y^2, [-1, 1, -0.5, 1.5]) But what if y is between -1 and x?

How can I plot something like ezsurf(x^2 + y^2, [-1, 1, -1, x])

Thank you.