Confusing documentation for clang-tidy bugprone-switch-missing-default-case

[Keith-S-Thompson]

The documentation for clang-tidy's "bugprone-switch-missing-default-case" option is confusing, incorrectly suggesting undefined behavior.

The following appears in clang-tools-extra/docs/clang-tidy/checks/bugprone/switch-missing-default-case.rst:

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.

The last sentence suggests that if no case is matched, the behavior is undefined. In fact, the behavior is well defined by the C standard, and the switch statement simply does nothing in these circumstances.

This documentation appears here: https://clang.llvm.org/extra/clang-tidy/checks/bugprone/switch-missing-default-case.html.

Suggested fix:

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
switch statement will do nothing and the program will continue
execution without any defined behavior or handling without
handling the value.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Keith Thompson (Keith-S-Thompson)

The documentation for clang-tidy's "bugprone-switch-missing-default-case" option is confusing, incorrectly suggesting undefined behavior.

The following appears in clang-tools-extra/docs/clang-tidy/checks/bugprone/switch-missing-default-case.rst:

> 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.

The last sentence suggests that if no case is matched, the behavior is undefined. In fact, the behavior is well defined by the C standard, and the switch statement simply does nothing in these circumstances.

This documentation appears here: <https://clang.llvm.org/extra/clang-tidy/checks/bugprone/switch-missing-default-case.html>.

Suggested fix:

> 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
> switch statement will do nothing and the program will continue
> execution without any defined behavior or handling without
> handling the value.

[Keith-S-Thompson]

The origin of this is an article posted today by user "pozz" in the comp.lang.c Usenet newsgroup.

Message-ID: <10da67g$3q59f$1@dont-email.me>

[carlosgalvezp]

Agreed, feel free to put up a PR with the suggested change!