  Isospin-1/2 $D\pi$ scattering amplitudes are computed using lattice QCD,
working in a single volume of approximately $(3.6\; \mathrm{fm})^3$ and with a
light quark mass corresponding to $m_\pi\approx239$ MeV. The spectrum of the
elastic $D\pi$ energy region is computed yielding 20 energy levels. Using the
L\"uscher finite-volume quantisation condition these energies are translated
into constraints on the infinite volume scattering amplitudes. For the first
time, we find a complex $D_0^\ast$ resonance pole from lattice QCD, strongly
coupled to the $S$-wave $D\pi$ channel, with a mass $m\approx 2200$ MeV and a
width $\Gamma\approx400$ MeV. Combined with earlier work investigating the
$D_{s0}^\ast$, and $D_0^\ast$ with heavier light quarks, similar couplings
between each of these scalar states and their relevant meson-meson scattering
channels are determined. The mass of the $D_0^\ast$ is consistently found well
below that of the $D_{s0}^\ast$, in contrast to the currently reported
experimental result.
