  A recent theory of the disorder-dependent slope of the upper critical field,
$H_{c2}$, at the superconducting transition temperature, $T_{c}$, is extended
to multiband superconductors aiming at iron-based superconductors, considering
two constant gaps of different magnitude and, potentially, different signs. The
result shows that there is only a narrow domain inside the $s_{\pm}$ pairing
state where the slope increases with the increase of transport (non-magnetic)
scattering rate, $P$. In most phase space, the slope should decrease in an
$s_{\pm}$ state and increase in the $s_{++}$ pairing state. The experiment
shows that in an archetypal iron-based superconductor,
$\textrm{Ba}_{1-x}\textrm{K}_{x}\textrm{Fe}_{2}\textrm{As}_{2}$ (BaK122),
non-magnetic disorder induced by electron irradiation increases the slope $S$
across the superconducting ``dome,'' at different $x$. This implies that
$\textrm{Ba}_{1-x}\textrm{K}_{x}\textrm{Fe}_{2}\textrm{As}_{2}$ is likely an
$s_{++}$ superconductor with two (or more) gaps of different magnitudes. This
work reopens a decade-long discussion of the nature of the superconducting
order parameter in iron pnictides.
