この挙動

bit.cs

using System;

class Program
{
    static void Main()
    {
        int i32 = 0;
        uint u32 = 0U;
        long i64 = 0L;
        ulong u64 = 0UL;


        var or00 = i32 | i32; //                     ok
        var or01 = u32 | u32; //                     ok
        var or02 = i64 | i64; //                     ok
        var or03 = u64 | u64; //                     ok

        var or10 = i32 | u32; //                     warning   long | long
        var or11 = (uint)i32 | u32; //               ok
        var or12 = (long)i32 | (long)u32; //         quick action to or10

        var or20 = i64 | u64; //                     error
        var or21 = (ulong)i64 | u64; //              ok

        var or30 = i64 | i32; //                     warning   long | long
        var or32 = i64 | (long)i32; //               quick action to or30
        var or33 = (ulong)i64 | (ulong)i32; //       warning
        var or34 = (ulong)i64 | (uint)i32; //        ok
        var or35 = (ulong)i64 | (ulong)(uint)i32; // quick action to or34

        var or31 = i64 | i32; //                     warning   long | long
        var or40 = i64 | u32; //                     ok        long | long
        var or50 = u64 | i32; //                     error
        var or60 = u64 | u32; //                     ok        ulong | ulong
    }
}

• 同じ型同士で定義されている
• 符号付き整数が拡大されることをすると警告
• 暗黙型変換ができないとエラー