pack reference elements bind to temporaries #463

fn::pack is an aggregate, so brace initialization can bind a reference element to a temporary: fn::pack<int const &>{42} compiles. For a named local initialized this way, the temporary lives as long as the pack. Returning the pack does not extend the temporary's lifetime, so the returned pack's element dangles.

#include <fn/pack.hpp>

template <typename T, typename... Args>
concept brace_constructible = requires(Args &&...args) { T{static_cast<Args &&>(args)...}; };
template <typename T, typename... Args>
concept spelled_lift = requires(Args &&...args) { fn::as_pack<T>(static_cast<Args &&>(args)...); };

// pack's braces bind a reference element to a temporary ...
static_assert(brace_constructible<fn::pack<int const &>, int>);
// ... which the spelled lift refuses, binding only an lvalue
static_assert(not spelled_lift<int const &, int>);
static_assert(spelled_lift<int const &, int &>);

// Compiles; the int dies when make returns, so reading the element of make() is a use-after-return.
auto make() -> fn::pack<int const &> { return fn::pack<int const &>{42}; }

int main() {}

Reading the element of make() is reported by AddressSanitizer as stack-use-after-return (gcc 16, with ASAN_OPTIONS=detect_stack_use_after_return=1).

Other reference APIs reject temporaries, following C++26 optional<T&>, reference alternatives in copack and choice (#451), as_pack with explicit element types, and append(std::in_place_type<T&>, ...). Brace initialization of pack still accepts them.

To reject temporaries, reference elements must be initialized through constructors. pack could gain constructors, or store reference elements behind a constructor that rejects temporaries, as copack does for reference alternatives. Either approach needs to account for aggregate initialization, structured bindings, triviality and constexpr.

This issue also blocks deprecating as_pack. #451 deprecates as_copack and as_choice in favour of naming the type, whose constructors refuse a temporary; the same advice for as_pack would replace as_pack<int const &>(x) with pack<int const &>{x}, which accepts what the lift refuses.

Related: #447.

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