On OSX I get

[sagelib-10.1.beta1]     sage/modular/arithgroup/farey.cpp:742:19: error: use of undeclared identifier 'bind'; did you mean 'find'?
[sagelib-10.1.beta1]                       bind(greater<int>(), _1, 0))/2;
[sagelib-10.1.beta1]                       ^~~~
[sagelib-10.1.beta1]                       find

Is that with Xcode "g++"?

Looking at the C++ doc, it may need to be std::bind to be fully correct. https://en.cppreference.com/w/cpp/utility/functional/bind

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Looking at the C++ doc, it may need to be std::bind to be fully correct. https://en.cppreference.com/w/cpp/utility/functional/bind

I now see there is using namespace std; at the beginning of the file, so not the problem.

Yes thats with the Xcode gcc, e.g. BigSur:

buildbot-sage@bigsur-osx build % gcc --version
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 13.0.0 (clang-1300.0.29.30)
Target: x86_64-apple-darwin20.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

A change to farey was done in #37646.

Rebased.
I don't know if this extra line in farey is necessary

And that's the bit Volker had problem with. I should redo the branch.

Rebased. I don't know if this extra line in farey is necessary

I would think it is, but it is only fatal if you are strict (which is a promised future behaviour).

I closed the issue but missed this PR. Fixed by 73e22d2.