Migrate ext::optional to std::optional (C++17 modernization)

[tobydrinkall]

Summary

Replaces the deprecated QuantLib::ext::optional compatibility shim with standard C++17 std::optional across the library, test suite, and build configuration, and removes the QL_USE_STD_OPTIONAL flag entirely.

Why: ql/optional.hpp mapped ext::optional to either boost::optional or std::optional behind the QL_USE_STD_OPTIONAL flag. The Boost branch carried a latent correctness landmine — the header literally #errors on Boost ≥ 1.91 because "Boost 1.91 introduced changes in boost::optional that silently changed the behavior of our code and would cause it to work incorrectly." In a pricing library, a dependency bump silently changing optional semantics can change results. This migration eliminates that risk permanently.

Why it's safe: QL_USE_STD_OPTIONAL already defaulted to ON, so ext::optional was already an alias for std::optional — the change is type-identical (same ABI), purely removing the indirection.

What changed (by layer)

ql/** library headers + sources ... 72 files   ext::optional → std::optional
test-suite/** .................... 9 files     ext::nullopt  → std::nullopt
build/config/shim ................ 9 files     #include <ql/optional.hpp> → <optional>

Mechanical replacements:

ext::optional<T>            -> std::optional<T>          (262 occurrences)
ext::nullopt               -> std::nullopt              (131 occurrences)
#include <ql/optional.hpp>  -> #include <optional>       (53 files)

ql/optional.hpp is reduced to a thin, deprecated #include <optional> (back-compat for downstream includers). The now-dead QL_USE_STD_OPTIONAL option is dropped from CMakeLists.txt, configure.ac, ql/userconfig.hpp, ql/config.hpp.cfg, the .ci/userconfig*.alt.hpp configs, and Docs/pages/config.docs.

Verification

• Build: 100% (clean CMake configure + build; generated config.hpp no longer defines QL_USE_STD_OPTIONAL).
• Test suite: 100% tests passed, 0 failed (full Boost.Test regression, ~285s) — i.e. pricing behavior unchanged.
• autoreconf -i succeeds (autotools config still valid).

Drafted as a demo artifact; commits are split into library / test-suite / build-flag-removal for reviewability.

Link to Devin session: https://app.devin.ai/sessions/d91f129a213241188cf4768afe9d6e1b
Requested by: @tobydrinkall

---

Devin Review

Status	Commit
⚪ Not started	—

Run Devin Review

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring

[devin-ai-integration]

Devin Review found 1 potential issue.

⚠️ 1 issue in files not directly in the diff

⚠️ CMakePresets.json still references removed QL_USE_STD_OPTIONAL option (CMakePresets.json:306)

The PR removes the QL_USE_STD_OPTIONAL CMake option from CMakeLists.txt (line 67 was deleted), but CMakePresets.json still sets "QL_USE_STD_OPTIONAL": "OFF" in two CI preset configurations (lines 306 and 326). These presets (linux-ci-build-with-nonstandard-options and windows-ci-build-with-nonstandard-options) were intended to test the library with boost::optional instead of std::optional. Since the option no longer exists in the build system, these settings silently become no-ops — CMake will create an unused cache variable without erroring. The CI builds using these presets no longer actually test a distinct configuration as intended.

View 3 additional findings in Devin Review.

[devin-ai-integration]

Addressed the Devin Review finding on CMakePresets.json in 77e091e: removed the stale "QL_USE_STD_OPTIONAL": "OFF" entries from the linux-ci-build-with-nonstandard-options and windows-ci-build-with-nonstandard-options presets, since the option no longer exists in the build system (they would have become silent no-op cache variables). grep -rn QL_USE_STD_OPTIONAL is now clean outside build dirs, and the file still parses as valid JSON.