Packed/extern unions are checked within comptime blocks, while they shouldn't

[DutchGhost]

According to the documentation https://ziglang.org/documentation/master/#Wrong-Union-Field-Access, the following should work, because extern and packed unions dont have the wrong field access check:

const MyUnion = packed union {
    n: usize,
    v: void,

    fn new() @This() {
        return MyUnion { .v = {} };
    }
};

test "this should work at compiletime" {
    comptime {
        var myUnion = MyUnion.new();

        var f = myUnion.n;
    }
}

However, when running zig test, it errors:

error: accessing union field 'n' while field 'v' is set
        var f = myUnion.n;

[Manuzor]

I also ran into this. The behavior is inconsistent between comptime and runtime.

const Foo = packed union {
    a: i8,
    b: u8,
};

test "runs fine" {
    var foo: Foo = .{ .a = -42 };
    try std.testing.expect(foo.b == 214);
}

test "does not compile" {
    const foo: Foo = .{ .a = -42 };
    // compile error: accessing union field 'b' while field 'a' is set
    try std.testing.expect(foo.b == 214);
}

zig version 0.8.0-dev.2275+8467373bb on Windows 10.0.19042.

[Vexu]

Works in correctly in the self-hosted compiler:

git grep "extern union doesn't trigger field check at comptime" test
test/behavior/union.zig:test "extern union doesn't trigger field check at comptime" {