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4437. constant_of(^^v) for variable v of array type produces reflection of pointer constant

Section: 21.4.7 [meta.reflection.queries] Status: New Submitter: Tomasz Kamiński Opened: 2025-10-29 Last modified: 2025-10-31

Priority: Not Prioritized

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Discussion:

The unintended consequence of the late change of reflect_constant to accept its parameter by value is that constant_of(a) returns a reflection of the value &a[0] for a variable of array type.

For reference, constant_of is specified as:

if constexpr (is_annotation(R))  {
  return C;
} else { 
  return reflect_constant([: R :]);
}

In case when [: R :] is a reference to array, it will decay to a pointer to the first element when accepted by value. I believe this is unintended and we should return an reflection of an array object instead.

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.7 [meta.reflection.queries] as indicated:

   consteval info constant_of(info r);
   

   -6 Let R be a constant expression of type info such that R == r is true. If r represents an annotation, then let C be its underlying constant.

   -7- Effects: Equivalent to:

   if constexpr (is_annotation(R)) {
     return C;
   } else if constexpr (is_array_type(type_of(R))) {
     return reflect_constant_array([: R :]);
   } else {
     return reflect_constant([: R :]);
   }