Add UDT support to value_ref via value_from

[Copilot]

Enables user-defined types (UDTs) to work in value_ref initializer lists by routing non-native types through value_from conversion.

Previously, UDTs with tag_invoke overloads for value_from_tag could not be used directly in array/object initializer lists because value_ref::from_const and from_rvalue only supported types with native value constructors.

Changes

• value_ref.hpp: Added is_native_value_type<T> trait identifying types with direct value constructors (JSON types, numerics, string-like, nullptr)
• impl/value_ref.hpp: Tag dispatch in from_const_impl and from_rvalue_impl to use value_from() for non-native types
• test/value_ref.cpp: Tests for UDTs in initializer lists, object values, and nested structures

Example

struct Point { int x, y; };

void tag_invoke(value_from_tag, value& jv, Point const& p) {
    jv = { {"x", p.x}, {"y", p.y} };
}

// Now works:
Point p{1, 2};
array arr{p, 123};           // UDT in initializer
object obj{{"point", p}};    // UDT as object value
value jv = {{"data", {p}}};  // Nested UDTs

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Streamlined construction and assignment, and from UDTs</issue_title>
<issue_description>I think we should explore getting rid of most of the constructors and assignment operators in json::value and replace them with this

template< class T >
value( T&& t );

template< class T >
value& operator=( T&& t );

and in the same exploration, allow value_ref to type erase UDTs. This will allow jv( t.a ) and jv = t.a and jv = vec to work.

The way to do this is to add another constrained constructor to value_ref which accepts UDTs when a suitable tag_invoke exists, and then type erase it by storing a function pointer which, when invoked, calls tag_invoke with the storage_ptr and produces the json::value.

</issue_description>

Comments on the Issue (you are @copilot in this section)

@vinniefalco @claude

• Fixes Streamlined construction and assignment, and from UDTs #1122

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.72%. Comparing base (64db9b7) to head (98ba3c6).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1128      +/-   ##
===========================================
+ Coverage    93.71%   93.72%   +0.01%     
===========================================
  Files           91       91              
  Lines         9153     9168      +15     
===========================================
+ Hits          8578     8593      +15     
  Misses         575      575              

Files with missing lines	Coverage Δ
include/boost/json/impl/value_ref.hpp	100.00% <100.00%> (ø)
include/boost/json/value_ref.hpp	100.00% <ø> (ø)

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