The exact solution of the diffraction problem for a plane electromagnetic wave at oblique incidence on a perfectly conducting half-plane was obtained more than 30 years ago. Recent advances in the high-frequency diffraction theory enable us to present the final solution in much simpler and suggestive forms. This solution is next used to test two uniform theories of edge diffraction. We find that uniform asymptotic theory (UAT) developed by Ahluwalia et al. recovers the exact solution, whereas uniform geometrical theory of diffraction (UTD) developed by Kouyoumjian and Pathak does not. The error of UTD is discussed.