[Bug analyzer/105273] -Wanalyzer-use-of-uninitialized-value warns on "missing" default for switch when callers can be statically determined

["avarab at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105273

--- Comment #2 from Ævar Arnfjörð Bjarmason <avarab at gmail dot com> ---
...To finish the report (Bugzilla's eager submitting threw me for a loop) the
issue is that while the analyzer is right in the *general* case about a
"switch" with a missing "default" being something that *could* be fed any
arbitrary value, in this case all of the possible values can be determined at a
compile-time.

Which is all this bug report is suggesting as an initial report, that it would
be nice to have that narrow case handled.

END OF NARROW REPORT

More generally though (and perhaps I should submit another report for this)
it's a really useful feature of GCC (and clang) that with C they put a bit of
trust in the programmer with -Wswitch (which is enabled under -Wall).

Because even though there are cases where the programmer is wrong and
exhaustively enumerating the enum labels isn't sufficient, in the general case
being able to avoid "default" cases in favor of exhaustively listing the labels
avoids a *lot* of subtle bugs in larger codebases.

That's because the values being thrown around to "switch" on are validated
already by [insert magic here], but whenever *new* values are added it's really
important to not miss 1/N switch statements that new labels need to be added
to.

In the testcase for this bug the compiler has enough visibility to determine
this to be true without the "[insert magic here]", but in cases where that's
not true it seems to me that those users -fanalyzer would be encouraged to add
"default" cases just to appease the compiler, and thus get worse warnings from
-Wswitch.

I may be missing something obvious, but it would be nice to have some way out
of that where you can have your cake & eat it too. I.e. only have -fanalyze
complain about this class of issue where -Wswitch would complain, and have the
current behavior in GCC 12.0 hidden behind some opt-in sub-flag of
-Wanalyzer-use-of-uninitialized-value.

Anyway, just my 0.02. Thanks a lot for -fanalyze, I've been trying it out on
the git codebase and it's uncovered a lot of genuine issues already. I'm just
filing some bugs for the long tail where the analyzer seemed to be
wrong/lacking. Thanks!