https://llvm.org/bugs/show_bug.cgi?id=25966
Bug ID: 25966
Summary: switch case with assert(false) causes
-Wimplicit-fallthrough warnings in both debug and
release builds
Product: new-bugs
Version: unspecified
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: cpeter...@mozilla.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
A not uncommon code pattern is to assert(false) in a switch statement's default
case but then fall through to a handled case.
Unfortunately, the assert(false) causes an "unannotated fall-through between
switch labels" warning in release (NDEBUG) builds and a "fallthrough annotation
in unreachable code" warning in DEBUG builds if you add a
[[clang::fallthrough]] annotation.
Possible workarounds:
1. Wrap the assert(false) in #ifdef DEBUG so there is no inert code in release
(NDEBUG) builds to cause an "unannotated fall-through between switch labels"
warning.
2. #define a special FALLTHROUGH_ASSERT() macro omits the unreachable
[[clang::fallthrough]] annotation in DEBUG builds. I took this approach in
Firefox, for the time being:
https://hg.mozilla.org/integration/mozilla-inbound/rev/d0a8d632dce5#l1.45
3. clang could allow a [[clang::fallthrough]] annotation after a call to a
noreturn function like abort(), but still warn about unreachable
[[clang::fallthrough]] annotations after an early break or return.
TEST CASE:
#include <assert.h>
int main(int argc, const char**)
{
// clang -Wimplicit-fallthrough -std=c++11 -DNDEBUG test.cpp
// warning: unannotated fall-through between switch labels
[-Wimplicit-fallthrough]
switch (argc) {
default:
assert(false); // assert on an unexpected switch value
case 0:
break;
}
// clang -Wimplicit-fallthrough -std=c++11 -DDEBUG test.cpp
// warning: fallthrough annotation in unreachable code
[-Wimplicit-fallthrough]
switch (argc) {
default:
assert(false); // assert on an unexpected switch value
[[clang::fallthrough]];
case 0:
break;
}
// Workaround #1:
switch (argc) {
default:
#ifdef DEBUG
assert(false); // assert on an unexpected switch value
#endif
case 0:
break;
}
// Workaround #2:
#ifdef DEBUG
#define FALLTHROUGH_ASSERT() assert(false)
#else
#define FALLTHROUGH_ASSERT() [[clang::fallthrough]]
#endif
switch (argc) {
default:
FALLTHROUGH_ASSERT(); // assert on an unexpected switch value
case 0:
break;
}
return 0;
}
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