[garethsb]

cpprest/json.h defines the private json::value constructor that takes a std::unique_ptr<details::_Value> inline, and the destructor is implicit. Both destroy the unique_ptr where _Value is incomplete. From C++23, std::unique_ptr's destructor is constexpr, and Clang 21 instantiates it there, so std::default_delete's static_assert fails in every translation unit that includes cpprest/json.h.

This PR declares the constructor and the destructor in json.h and defines them in json.cpp, where _Value is complete. The change is taken from vcpkg's ports/cpprestsdk/fix-incomplete-json-value.patch, added by Nicholas Matteo in microsoft/vcpkg#51307, and keeps him as the commit author. The one difference from the patch is where the destructor is declared: here it is with the other public special members, not in a public: block among the private members. json::value already declares its copy and move operations, so declaring the destructor leaves them unchanged.

Testing

I tested with Clang 21.1.8 on Ubuntu 26.04:

• A file that includes cpprest/json.h fails at -std=c++23 without this change, with both libstdc++ 15 and libc++ 21. It compiles with this change at C++17, C++20 and C++23.
• With -DCMAKE_CXX_STANDARD=23, the library build fails without this change and succeeds with it. The library's CMake adds -std=c++11, so only a consumer's C++23 setting triggers the error.
• The default build passes all 7 test suites.

Not Included

Issue #13 also asks for a CI job with a recent Clang, so this PR doesn't close it. A C++23 job can't build the tests yet, because three test files don't compile at C++20 or later:

• test_runner.cpp uses requires as a variable name, and requires is a keyword from C++20.
• strings.cpp and memstream_tests.cpp write char16_t values to a stream through UnitTest++, and C++20 deletes those stream operators.

Part of #13.

Co-authored-by: Gareth's 🤖 buddy