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4483. Multidimensional arrays are not supported by meta::reflect_constant_array and related functions

Section: 21.4.3 [meta.define.static] Status: New Submitter: Tomasz Kamiński Opened: 2025-11-27 Last modified: 2025-12-05

Priority: Not Prioritized

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Discussion:

As any array type (even of structural types) is not considered an structural type, per 13.2 [temp.param] p12, any invocation of reflect_constant_array/define_static_array with a multidimensional array or span of arrays is ill-formed due to the Mandates in 21.4.3 [meta.define.static] p8 that requires range value type to be structural.

As a consequence, constant_of currently supports only single-dimensional arrays (reflect_constant_array strips outermost extents), while multi-dimensional arrays are rejected.

Furthermore, define_static_object currently uses define_static_array(span(addressof(t), 1)).data(), for array types. Since for T[N] input this creates an multidimensional T[1][N] constant parameter object, this function does not support arrays at all. Creating a distinct template parameter object leads to emission of (otherwise unnecessary) additional symbols, and breaks the invariant that for all supported object types &constant_of(o) == define_static_object(o). We should use reflect_constant_array for arrays directly.

The Throws clause of reflect_constant_array was updated to include any exception thrown by iteration over range.

[2025-12-05; LWG telecon. Wording tweaks]

Proposed resolution:

This wording is relative to N5014 amended with changes from LWG 4432⁽ⁱ⁾.

1. Modify 21.4.3 [meta.define.static] as indicated:

   template<ranges::input_range R>
     consteval info reflect_constant_array(R&& r);
   

   -8- Let T be ranges::range_value_t<R> and e_i be static_cast<T>(*iti), where it_i is an iterator to the i^th element of r.

   -9- Mandates:

   • T is a structural type (13.2 [temp.param]), is_constructible_v<T, ranges::range_reference_t<R>> is true, and
   • T satisfies copy_constructible

   -10- Let V be the pack of values of type info of the same size as r, where the i^th element is

   • reflect_constant(ei)

   .

   -11- Let P be

   • (11.1) — If sizeof...(V) > 0 is true, then the template parameter object (13.2 [temp.param]) of type const T[sizeof...(V)] initialized with {[:V:]...}.
   • (11.2) — Otherwise, the template parameter object of type const array<T, 0> initialized with {}.

   -12- Returns: ^^P.

   -13- Throws: Any exception thrown by the evaluation of any e_i, or meta::exception if evaluation of any reflect_constant(ei) would exit via an exception.

   […]

   template<class T>
     consteval const remove_cvref_t<T>* define_static_object(T&& t);
   

   -15- Effects:Equivalent to:

   using U = remove_cvref_t<T>;
   if constexpr (meta::is_class_type(^^U)) {
     return addressof(meta::extract<const U&>(meta::reflect_constant(std::forward<T>(t))));
   
   } else {
     return define_static_array(span(addressof(t), 1)).data();
   }