stage2: Bitcast allowed between types of different sizes

[topolarity]

Zig Version

0.10.0-dev.4430+99c3578f

Steps to Reproduce

test {
    const T = extern struct{ i: u64 = 0, x: u16 = 0}; 
    var x = T{};
    //_ = @bitCast(u128, x); // error on stage2
    _ = @bitCast(u80, x); // error on stage1
}

Expected Behavior

I'd expect the @bitCast to be with respect to the actual layout size of the struct, not the sum of the packed size of its fields.

Actual Behavior

Cast is allowed to u80, but not u128.