Is the unique solution of autonomous ODE infinitely differentiable

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Consider the ODE \begin{align} y'&=f(y)\\ y(0)&=y_0 \end{align} where $f$ is Lipschitz and, if it matters, always positive. When restricted on an interval $[0,a]$, is the unique solution of such ODE infinitely differentiable? If not, under which assumptions would this be true?