Unpacker.readUnion returns !?T but unpackUnion returns !T, so it never compiles

[acoustid-bot]

Unpacker.readUnion promises one more level of optional than unpackUnion returns, so it cannot be instantiated with any type argument.

// src/msgpack.zig:232
pub fn readUnion(self: Unpacker, comptime T: type) !?T {
    return unpackUnion(self.reader, self.allocator, T);
}

unpackUnion returns !T (src/union.zig:262) — it already handles the optional case internally, via NonOptional(T) and the ?u16 map-header path. Wrapping the result in another ? makes every instantiation a type error:

• readUnion(U) → "error union payload 'U' cannot cast into error union payload '?U'"
• readUnion(?U) → "optional type child 'U' cannot cast into optional type child '?U'"

Reproducer

const U = union(enum) { a: u8 };

test "readUnion" {
    const d = [_]u8{ 0x81, 0xa1, 'a', 0x01 };
    var r = std.Io.Reader.fixed(&d);
    _ = try msgpack.unpacker(&r, std.testing.allocator).readUnion(U);
}

Fails to compile against main (Zig 0.16.0). Substituting ?U fails too.

Suggested fix

Drop the extra optional, matching how readStruct already forwards to unpackStruct:

pub fn readUnion(self: Unpacker, comptime T: type) !T {
    return unpackUnion(self.reader, self.allocator, T);
}

Callers wanting the nullable behaviour then pass ?U explicitly, which unpackUnion already supports.

Scope

I exercised every other method on Packer and Unpacker once each; this and readArray (#21) are the only two that are uninstantiable. Everything else compiles and round-trips. Same root cause for CI missing it: refAllDecls does not instantiate generics.