Related:

• Proposal: Rename packed keyword to bitpack #12852
• make backing type of packed struct and packed union mandatory #24715
• Proposal: make packed undefined semantics consistent with those of integers #25279
• do not allow enum/packed struct/packed union in extern positions if backing type is implicit #24714

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

const U = packed union(u16) {
    a: u16,
    b: packed struct(u16) {
        a: u7,
        b: u9,
    },
};

test "packed union with explicit integer type" {
    const x: u16 = 1234;
    const u: U = @bitCast(x);
    try expect(u.a == 1234);
    try expect(u.b.a == 82);
    try expect(u.b.b == 9);
}

Currently gives:

test.zig:4:24: error: packed union does not support enum tag type
const U = packed union(u16) {
                       ^~~

Instead, this test should pass.

Semantics Clarifications

When combined with #24714:

• just like enums, the type has well-defined memory layout if and only if explicit backing integer provided
• just like enums, the type is allowed in extern contexts if and only if explicit backing integer provided
• just like enums, the type is allowed in switch expressions independent of explicit backing integer existing
• just like enums, the type supports equality independent of explicit backing integer existing