[clang-tidy][NFC] Clarify switch-missing-default-case doc (#164699)

[Keith-S-Thompson]

Falling through a defaultless switch statement has well defined behavior. Fixes #164699.

Credit for noticing this problem goes to user "pozz" on comp.lang.c,
Message-ID: 10da67g$3q59f$1@dont-email.me

[github-actions]

Thank you for submitting a Pull Request (PR) to the LLVM Project!

This PR will be automatically labeled and the relevant teams will be notified.

If you wish to, you can add reviewers by using the "Reviewers" section on this page.

If this is not working for you, it is probably because you do not have write permissions for the repository. In which case you can instead tag reviewers by name in a comment by using @ followed by their GitHub username.

If you have received no comments on your PR for a week, you can request a review by "ping"ing the PR by adding a comment “Ping”. The common courtesy "ping" rate is once a week. Please remember that you are asking for valuable time from other developers.

If you have further questions, they may be answered by the LLVM GitHub User Guide.

You can also ask questions in a comment on this PR, on the LLVM Discord or on the forums.

[llvmbot]

@llvm/pr-subscribers-clang-tools-extra

@llvm/pr-subscribers-clang-tidy

Author: Keith Thompson (Keith-S-Thompson)

Changes

Falling through a defaultless switch statement has well defined behavior.

Credit for noticing this problem goes to user "pozz" on comp.lang.c,
Message-ID: <10da67g$3q59f$1@dont-email.me>

---

Full diff: https://github.com/llvm/llvm-project/pull/164709.diff

1 Files Affected:

• (modified) clang-tools-extra/docs/clang-tidy/checks/bugprone/switch-missing-default-case.rst (+2-2)

diff --git a/clang-tools-extra/docs/clang-tidy/checks/bugprone/switch-missing-default-case.rst b/clang-tools-extra/docs/clang-tidy/checks/bugprone/switch-missing-default-case.rst
index 648c2c208a4ec..3ce862ff8afcc 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/bugprone/switch-missing-default-case.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/bugprone/switch-missing-default-case.rst
@@ -9,8 +9,8 @@ on covering cases with non-enums where the compiler may not issue warnings.
 Switch statements without a default case can lead to unexpected
 behavior and incomplete handling of all possible cases. When a switch statement
 lacks a default case, if a value is encountered that does not match any of the
-specified cases, the program will continue execution without any defined
-behavior or handling.
+specified cases, the switch statement will do nothing and the program will
+continue execution without handling the value.
 
 This check helps identify switch statements that are missing a default case,
 allowing developers to ensure that all possible cases are handled properly.

[vbvictor]

In fact, the behavior is well defined by the C standard, and the switch statement simply does nothing in these circumstances.

It is defined by C standard, but does it defined by C++ standard? I've seen a recent example https://discourse.llvm.org/t/ub-when-type-punning-through-unions/88527 of code that well-defined in C but UB in C++.

[vbvictor]

LGTM, regardless of #164709 (comment).

[Keith-S-Thompson]

Yes, the defined behavior is the same in C++ as it is in C.

Quoting a draft of the ISO C++ standard, section [stmt.switch], emphasis added:

When the switch statement is executed, its condition is evaluated and compared with each case constant. If
one of the case constants is equal to the value of the condition, control is passed to the statement following
the matched case label. If no case constant matches the condition, and if there is a default label, control
passes to the statement labeled by the default label. If no case matches and if there is no default then none
of the statements in the switch is executed.

[github-actions]

@Keith-S-Thompson Congratulations on having your first Pull Request (PR) merged into the LLVM Project!

Your changes will be combined with recent changes from other authors, then tested by our build bots. If there is a problem with a build, you may receive a report in an email or a comment on this PR.

Please check whether problems have been caused by your change specifically, as the builds can include changes from many authors. It is not uncommon for your change to be included in a build that fails due to someone else's changes, or infrastructure issues.

How to do this, and the rest of the post-merge process, is covered in detail here.

If your change does cause a problem, it may be reverted, or you can revert it yourself. This is a normal part of LLVM development. You can fix your changes and open a new PR to merge them again.

If you don't get any reports, no action is required from you. Your changes are working as expected, well done!