When a W⁺ boson decays into a positron and a neutrino, lepton number is conserved because the W⁺ boson is a non-lepton, the neutrino is a lepton, and the positron is an antilepton. Likewise, when a W⁻ boson decays into an electron and an antineutrino, lepton number is conserved because the W⁻ boson is a non-lepton, the electron is a lepton, and the antineutrino is an antilepton.
Source: wiktionary