These small modifications allow building Tesseract 5 with clang-7 on Debian stretch. They are not sufficient for g++ 6.3 (see issue #3255).

This happens when no C++17 available in stdlib.
Do we require C++17 support still?

(Compilers port some core language (not std) features into older standards. Like such if constexpr into c++14, c++11 and probably even c++98.)

configure accepts g++ and clang++ on Debian stretch, so both compilers should formally support C++17. Nevertheless both fail to compile the current code because their C++17 support has deficits.

Therefore I think that it's good to check for C++17 to get at least a compiler which nearly supports it. If we would lower the requirements and only check C++14, we might risk getting more reports for old compilers which are nearly compatible with C++14.

Maybe we also want to use more C++17 features (which may then break builds with clang from Debian stretch, too) in later 5.x.y releases.

5.x.x releases won't get too much attention and code changes, so they could live with current "semi-C++17" standard.

As always I propose to move forward with the next 6,7... main branch (C++20/23).

By the way, do we have v5 branch already? Such fixes could land exactly there.
But main will keep moving forward.

Before applying this patch, did you try to install libc++-7-dev, and tell clang to use it: clang-7 -std=c++17 -stdlib=libc++-7(or maybe it should be -stdlib=libc++7, I don't remember...) ?

Also note that Debiain Stretch is not maintained by the Debian project anymore.

https://wiki.debian.org/LTS

Debian Long Term Support (LTS) is a project to extend the lifetime of all Debian stable releases to (at least) 5 years. Debian LTS is not handled by the Debian security team, but by a separate group of volunteers and companies interested in making it a success.

EOL of Debian Stretch LTS: June 30, 2022.

Personally, I don't think we should support it in 5.0.0.

Compilers port some core language (not std) features into older standards. Like such if constexpr into c++14, c++11 and probably even c++98.

https://gcc.gnu.org/gcc-12/changes.html

New warning options for C++ language mismatches: -Wc++11-extensions, -Wc++14-extensions, -Wc++17-extensions, -Wc++20-extensions, and -Wc++23-extensions. They are enabled by default and can be used to control existing pedwarns about occurences of new C++ constructs in code using an old C++ standard dialect.

These warnings are also available in Clang:

https://clang.llvm.org/docs/DiagnosticsReference.html#wc-17-extensions