Can't use ?*Self as a field type when it is passed to/returned from a function

[chadwain]

Using 0.6.0+1a4059ed8, this works:

const S = struct {
    a: ?*S,
};

pub fn main() void {
    _ = @as(S, undefined);
}

but this doesn't:

fn identity(comptime T: type) type {
    return T;
}

const S1 = struct {
    a: identity(?*S1),
};

fn addOptionalPointer(comptime T: type) type {
    return ?*T;
}

const S2 = struct {
    a: addOptionalPointer(S2),
};

const S3 = struct {
    a: @import("std").ArrayList(?*S3),
};

pub fn main() void {
    _ = @as(S1, undefined);
    _ = @as(S2, undefined);
    _ = @as(S3, undefined);
}

./main.zig:5:12: error: struct 'S1' depends on itself
const S1 = struct {
           ^
./main.zig:6:17: note: referenced here
    a: identity(?*S1),
                ^
./main.zig:6:16: note: referenced here
    a: identity(?*S1),
               ^
./main.zig:13:12: error: struct 'S2' depends on itself
const S2 = struct {
           ^
./main.zig:9:46: note: referenced here
fn addOptionalPointer(comptime T: type) type {
                                             ^
./main.zig:14:26: note: referenced here
    a: addOptionalPointer(S2),
                         ^
./main.zig:17:12: error: struct 'S3' depends on itself
const S3 = struct {
           ^
./main.zig:18:33: note: referenced here
    a: @import("std").ArrayList(?*S3),
                                ^
./main.zig:18:32: note: referenced here
    a: @import("std").ArrayList(?*S3),
                               ^

(Trying the same thing with only ordinary pointers seems to work fine.)
Related: #5414

[Vexu]

Duplicate of #4562.