[azerbenali]

Fixes #1263

what() now returns the error when it converts to std::string, otherwise the old message. Kept C++11-compatible (tag dispatch instead of if constexpr) since the header targets C++11, and the message is in a shared_ptr so copying the exception doesn't allocate.

Note: with C++23 nonstd::expected maps to std::expected, which this doesn't change.

[facontidavide]

Thanks @azerbenali, this is a real usability win: getInput<T>(...).value() failing with a bare bad_expected_access is a common complaint.

I pushed one commit (dcea354) on top of a merge of master that simplifies it. m_error already holds the string, so what() can return m_error.c_str() through an overload on std::string, with a template fallback for other error types. The shared_ptr member isn't needed: copying the exception copies m_error anyway. Without the extra member the layout of this public template class stays unchanged. The // BT.CPP patch marker flags the edit to the vendored header for whoever re-vendors it. Your tests pass unchanged (598/598 locally). I'll squash-merge once CI is green.

[azerbenali]

Thanks Davide! Appreciate the improvements and your help!