The following reaction is known as 

${\mathrm{ArN}}_2^{+} {\mathrm{X}}^{-} +\mathrm{Cu}/\mathrm{HCl}\to \mathrm{Ar}-\mathrm{Cl} +{\mathrm{N}}_2 +\mathrm{CuX}$

Gattermann reaction is the reaction in which benzene diazonium chloride reacts with $\mathrm{Cu}/\mathrm{HCl}$ to form chlorobenzene and this ${\mathrm{ArN}}_2 + \mathrm{Cl}$ when reacts with $\mathrm{Cu}/\mathrm{HBr}$ forms bromobenzene. Thus, the given reaction scheme is known as the Gattermann reaction.