Incorrect error inference check on generated types

[tgschultz]

const Struct = struct {
    pub fn func() (error{})!usize {
        return 0;
    }
};

fn AnotherStruct(comptime SubStruct: type) type {
    return struct {
        fn anotherFunc() !void {
            _ = try SubStruct.func();
        }
    };
}

test "incorrect error inference" {
    const GeneratedStruct = AnotherStruct(Struct);
    try GeneratedStruct.anotherFunc();
}

error: function with inferred error set must return at least one possible error
        fn anotherFunc() !void

This can be worked around by declaring the return type of anotherFunc as @typeOf(SubStruct.func).ReturnType.ErrorSet!void

[andrewrk]

I'm going to defer this bug fix as I'm still considering what the behavior should be for cases like these when there is an explicit empty error set or an inferred empty error set.