  Let $M$ be a compact smooth manifold equipped with a positive smooth density
$\mu$ and $H$ be a smooth distribution endowed with a fiberwise inner product
$g$. We define the Laplacian $\Delta_H$ associated with $(H,\mu,g)$ and prove
that it gives rise to an unbounded self-adjoint operator in $L^2(M,\mu)$. Then,
assuming that $H$ generates a singular foliation $\mathcal F$, we prove that,
for any function $\varphi$ from the Schwartz space $\mathcal S(\mathbb R)$, the
operator $\varphi(\Delta_H)$ is a smoothing operator in the scale of
longitudinal Sobolev spaces associated with $\mathcal F$. The proofs are based
on pseudodifferential calculus on singular foliations developed by
Androulidakis and Skandalis and subelliptic estimates for $\Delta_H$.
