Seemingly random values from @bitCast(u64, packed_struct)

[SamTebbs33]

const expectEqual = @import("std").testing.expectEqual;

const S1 = packed struct {
    a: u8,
};

const S2 = packed struct {
    a: u4,
};

const S = packed struct {
    int: u52,
    s1: S1,
    s2: S2,
};

fn makeStruct(s1: S1, s2: S2) S {
    return S{
        .int = 0,
        .s1 = S1{
            .a = s1.a,
        },
        .s2 = S2{ .a = s2.a },
    };
}

test "reproduce" {
    const actual = makeStruct(S1{ .a = 0 }, S2{ .a = 0 });
    // Arbitrary value, not important for this test case
    const expected: u64 = 0;
    expectEqual(expected, @bitCast(u64, actual));
}

Each time this is run with zig test reproduce.zig it has a different value for actual. It doesn't happen if actual is created in-place rather than with a function, so could that mean it has something to do with result location semantics? It doesn't happen if S1.a and S2.a are assigned to 0 directly rather than from the members of the structs given as arguments. I couldn't reproduce this before 5b27943

[nektro]

this works now

[Vexu]

test/behavior/packed-struct.zig contains tests similar enough to this.