https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96400
Bug ID: 96400
Summary: False positive on Wunused-but-set-variable for static
constexpr var used in lambda
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: gccbugbjorn at fahller dot se
Target Milestone: ---
Similar to Bug 96311, but not quite the same.
This code is flags "is_zero" as being unused but set:
template <typename Pred, typename Val>
bool test(Pred p, Val v)
{
return p(v);
}
bool func(int* p)
{
static constexpr auto is_zero = [](auto v) { return v == 0;};
return test([](auto v){return is_zero(*v);}, p);
}
Output:
<source>: In function 'bool func(int*)':
<source>:9:27: warning: variable 'is_zero' set but not used
[-Wunused-but-set-variable]
9 | static constexpr auto is_zero = [](auto v) { return v == 0;};
| ^~~~~~~
"is_zero" can correctly be used in the lambda without being captured, since
it's static. The warning only appears when using the extra indirection
"test()". Calling the anonymous lambda directly, without the indirection
"test()", does not display the warning.
Possibly "constexpr" is a red herring above.
Godbolt link:
https://godbolt.org/z/x4zb3s
