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Fix C++17 support - #702

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RobLoach wants to merge 5 commits into
ChaiScript:developfrom
RobLoach:fixes-for-c17
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Fix C++17 support#702
RobLoach wants to merge 5 commits into
ChaiScript:developfrom
RobLoach:fixes-for-c17

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@RobLoach

@RobLoach RobLoach commented May 2, 2026 •
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This attempts to fix some of the C++17 compatibility. ChaiScript is advertised as C++17 compatible, so we should either do that, or move to C++20.

@lefticus lefticus mentioned this pull request Aug 12, 2026
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@lrosenthol

lrosenthol commented Aug 13, 2026

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For context: we independently wrote our own C++17-compatibility shims for the same set of issues this PR addresses (std::span, std::type_identity_t, string_view::starts_with/ends_with, std::ssize/parser pointer arithmetic, and a couple of dependent-name typename fixes). Comparing the two approaches side by side turned up a few things that might be useful here:

Issue This PR (#702) Our independent fix
std::span (C++20-only) Re-adds the old pre-span Function_Params class as the C++17 fallback (constexpr, begin/end pointers). Normalizes call sites to Function_Params{&x, &x+1} (begin, end) so behavior matches in both modes. New minimal fallback with a generic SFINAE'd container constructor and a (ptr, count) constructor, mirroring std::span's primary constructor. Call sites use Function_Params{&x, 1} (ptr, count) instead. Also guards with _MSVC_LANG >= 202002L in addition to __cplusplus, since MSVC won't report __cplusplus correctly without /Zc:__cplusplus — worth adding here too, otherwise the C++17 fallback path may silently be taken (or not) incorrectly on MSVC.
std::type_identity_t Defines type_identity/type_identity_t inside namespace std, which is technically UB (users can't add non-specialization declarations to std::). Defines it in chaiscript:: instead — avoids the namespace pollution.
string_view::starts_with/ends_with #if-branches between the real methods (C++20) and a hand-written compare()-based version (C++17). Same compare()-based logic, but written once, unconditionally — valid under both standards without the #if branch.
std::ssize/std::next in the parser #if-branches between std::ssize (C++20) and (int)std::size (C++17). std::next(begin, static_cast<ptrdiff_t>(size)) unconditionally, sidestepping the need for std::ssize entirely.

Happy to share the full internal diff if useful, but figured the comparison itself might be worth having on this thread.

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@RobLoach @lrosenthol