fix(cpp): support std::optional in Utf16_Utf8 string conversion

[schani]

Bug

With --wstring use-wstring --no-boost (the C++17 std::optional code path) and an optional string property, the generated C++ fails to compile:

testy2.h: In instantiation of 'static toType quicktype::Utf16_Utf8<fromType, toType>::convert(fromType) [with fromType = std::optional<std::string>; toType = std::optional<std::wstring>]':
testy2.h:165:94:   required from here
testy2.h:98:62: error: could not convert '...convert(...)' from 'optional<basic_string<char>>' to 'optional<basic_string<wchar_t>>'

Root cause

The generated Utf16_Utf8 helper class (emitted in emitHelperFunctions, packages/quicktype-core/src/language/CPlusPlus/CPlusPlusRenderer.ts) only had convert overloads for std::shared_ptr, std::vector, and std::map. There was no overload for std::optional/boost::optional, so calls on optional string properties fell through to the generic fallback overload, which just returns the value unchanged — producing a type mismatch the compiler rejects.

Fix

Added a recursive convert overload for the configured optional type (std::optional or boost::optional, matching the --wstring/--no-boost renderer options) to Utf16_Utf8, mirroring the existing shared_ptr overload. It's only emitted when the generated code actually has optional properties (haveOptionalProperties), matching how other optional-only helper code is conditionally emitted elsewhere in this renderer.

Test coverage

Pinned the existing C++ wstring: use-wstring quicktest renderer option to test/inputs/json/priority/bug2521.json, which contains an optional string property (foo present on only one array element). This exercises the Utf16_Utf8<std::optional<...>>::convert path at compile time for the use-wstring code path, previously it ran against unpinned default quicktest inputs that didn't hit this combination.

Verification

• Confirmed the bug on unfixed master using the repro from the issue/triage comment (schema2.json + --wstring use-wstring --no-boost ...): g++ 13.3 -std=c++17 fails with the exact error quoted above.
• After the fix, regenerated the same repro and compiled with g++ -std=c++17 -I. -c main2.cpp — compiles cleanly.
• Generated C++ for bug2521.json with --wstring use-wstring, built it against the real test/fixtures/cplusplus driver (main.cpp/json.hpp) with g++ -O0 -std=c++17 — compiles and runs successfully (previously this exact combination is what the pinned quicktest now exercises in CI).
• npm run build passes.
• npm run test:unit — 163/163 tests pass.
• Ran QUICKTEST=true FIXTURE=cplusplus script/test locally; it hit an unrelated, pre-existing environment gap (this sandbox has no libboost-dev, so the pre-existing ["pokedex.json", { boost: "true" }] quicktest fails with boost/optional.hpp: No such file or directory before reaching later quicktests) — this is unrelated to this change and will run fully in CI, which has boost available.

Fixes #2383

🤖 Generated with Claude Code