expressing $g=\frac{df}{dy}$ with $y=y(x)$ as $g(x)$ instead of $g(y)$ in MATLAB

Question

I couldn't find anything related to functionalDerivative on StackExchange, but I apologise if this is a duplicate of another question.

Suppose I have a function $f(y)$, where $y$ is itself a function of $x$. Using MATLAB, I'd like to find $g(x)$, where $g=\frac{∂f}{∂y}$ as a function of $x$.

Here's my code, which returns an error:

syms y(x)
f = 2*y^2;
y = x+3;
g = functionalDerivative(f,y)

If I instead define $y$ after calculating $g$, I obtain $g(y)$ instead of $g(x)$:

syms y(x)
f = 2*y^2;
g = functionalDerivative(f,y);
y = x+3;
simplify(g)

I'm pretty new to MATLAB, so maybe I'm missing something really simple.

Thanks for any help.