Lint: Lint for std::for_each_n

[KBaichoo]

Signed-off-by: Kevin Baichoo kbaichoo@google.com

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Commit Message: Lint: Lint for std::for_each_n
Additional Description: It's insufficiently supported in C++17. See #14923
Risk Level: low
Testing: unit test
Docs Changes: NA
Release Notes: NA
Platform Specific Features: NA

[antoniovicente]

Thanks for addressing the compile issues introduced by #14923

Change looks good if we decide to avoid std::for_each_n for the time being.

[antoniovicente]

cc @jrajahalme

[lizan]

I'm generally not in favor of this kind of limitation because of it is not well supported in older version of stdlib (libstdc++ <= 8). We can bump build requirement if it doesn't cause a lot of churn.

[jrajahalme]

@lizan I agree that limiting use of a standardized C++17 feature is not ideal, but for_each_n is available only on the newest libc++. However, I'd like to retain compatibility with GCC on Ubuntu 18.04 for now. GCC due to working cross-compilation toolchain we have for arm64 (I have no idea how to do that on clang), and Ubuntu 18.04 for ability to run the same Envoy binary in Ubuntu 18.04 and 20.04 based images. Unfortunately for_each_n does not work on GCC 7 that is the system compiler for Ubuntu 18.04.

[jrajahalme]

To clarify, bumping build requirements is a potential concern for projects building on Envoy, not just Envoy itself. There is already some churn due to Envoy CI apparently using only a bleeding edge clang setup :-)

[mattklein123]

Thanks!