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I wonder if there's some clever, automatic way of knowing if a particular compiler warning (e.g. -Wunused-parameter) comes from the group -Wall, -Wextra, or another group, for both GCC and Clang.

Use case: we want to enable:

-Wall -Wextra -pedantic

However, some of the pedantic warnings are unapplicable to us and we want to disable them, example:

-Wall -Wextra -pedantic -Wno-c++20-designator

Now, we want to be 100% sure that we are not disabling anything from -Wall nor -Wextra. How do we ensure that -Wc++20-designator is not part of either? Of course one can go and check the documentation, but this is a tedious process when you have many such warnings or when you upgrade the compiler and get new warnings.

Our use case to ensure that all -Wall, -Wextra warnings will always be active, regardless of the disabled warnings from -pedantic.

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  • Please one question per question. Automatically know if a GCC/Clang warning comes from Wall or Wextra? is not an alternative to is there a way to ensure that all -Wall, -Wextra warnings will always be active, regardless of the disabled warnings?. I also do not understand the second question, yes, there is a way, do not disable any warnings. Commented Nov 30, 2021 at 12:44
  • "we want to be 100% sure that we are not disabling anything from -Wall nor -Wextra". That sounds like a management feature and I am wondering why pedantic is not the minimum. Which warning is enabled in which state is listed in the gcc docs. But I believe it will change from time to time and maybe the doc will not change always. I personally like to see every warning and fix as soon as possible. Which warning is switched on by the flags can be seen here: gcc.gnu.org/onlinedocs/gcc/Warning-Options.html. We compile our unit test targets with ALL warnings and count them. >0 is fault! Commented Nov 30, 2021 at 12:45
  • @KamilCuk that's not an answer to the question. Klaus "which one is the minimum" is out of this discussion. I'm aware of the GCC link, even though there's not an equivalent one for Clang (you need to do more detective work). I'll update the question in this regard. My question is if this can be done automatically/programatically instead of manually going into the docs every time. Commented Nov 30, 2021 at 12:54
  • Updated question to be formulated as only 1 question. Commented Nov 30, 2021 at 12:56
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    You can ask gcc what is enabled by which option, try gcc -Q -Wall --help=warnings | grep enabled. Would that help you? Commented Nov 30, 2021 at 13:02

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There's no specific command to get a direct answer to the question "which warning group does a given warning come from?", but it's possible to infer this information automatically by querying the compiler which warnings are enabled and checking if the warning we are interested in is part of the list of enabled warnings.

This is done as follows:

  • GCC: gcc -Q -Wall --help=warnings | grep enabled | grep unused-parameter (credits to @dratenik).
  • Clang: this functionality is not baked into the compiler, unlike GCC, so we need to use the diagtool tool: diagtool show-enabled -Wall foo.cpp | grep unused-parameter.

Bonus: Unrelated to the original question but related to the original use case: to be sure that all of Wall and Wextra are enabled, regardless of which warnings are disabled, the solution (only works for Clang) is to put Wall Wextra at the very end:

clang -Wno-unused-parameter -Wall -Wextra

In this case, if we accidentally disable the unused-parameter warning, Wall will be applied after, re-enabling the warning.

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This tool parses the warnings in GCC and Clang and how they are grouped, and the output (for every version) is also added to the github. The warning "tree" shows the hierarchy of what is under -Wall, etc. It may be possible for a warning to be under multiple parents.

For example, Clang 21:

https://github.com/pkolbus/compiler-warnings/blob/main/clang/warnings-top-level-21.txt

It shows -Wc++20-designator is under -Wc++2a-extensions, -Wc99-extensions, and -Wpedantic .

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