Should & keep lvalue-reference payloads of optional<T&>? #449

operator& copies out of lvalue-reference payloads. On main, optional<int&> & optional<int> is optional<pack<int, int>> and optional<int&> & optional<long&> is optional<pack<int, long>>: each factor is the carrier's value_type, which is T for optional<T&>.

The change for #448 makes the disjunction keep reference payloads: optional<int&> | optional<long&> becomes optional<copack<int&, long&>>. After it, | and & treat the same operands differently. This issue asks whether & should also take its factors from the payload type, so that optional<int&> & optional<int> is optional<pack<int&, int>>.

For keeping references

• pack admits lvalue-reference elements, and lifting an lvalue keeps it: for int x, as_pack(x) is pack<int&>.
• The disjunction keeps them after Allow lvalue-reference alternatives in copack #448, so both operators would agree on what a reference payload contributes.
• just<T&> (Support lvalue-reference payloads in just #437) follows whatever rule optional<T&> follows.

For copying

• Appending a plain value to a product copies it: as_pack(x) & y is pack<int&, long>. A reference enters a product only through an explicit lift, and a carrier operand could be read as a plain value.

Affected either way

The referent's cv-qualification leaks into the product: optional<int const&> & optional<long&> is optional<pack<int const, long>>. Keeping references gives pack<int const&, long>; copying should give pack<int, long>.

Assisted-by: Claude:claude-opus-5-5