https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105560
Bug ID: 105560
Summary: Spurious -Wunused-local-typedefs warning on a typedef
used on returned type
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: redbeard0531 at gmail dot com
Target Milestone: ---
https://godbolt.org/z/zcY11ezev
#include <type_traits>
#include <utility>
template <typename F>
typename F::Out call(F&& fun) {
typename F::Out var = fun(1);
return var;
}
void test() {
// Fill builder with bson-encoded elements from view.
auto decorateLambda = [](auto&& lambda) {
using Lambda = std::remove_reference_t<decltype(lambda)>;
struct DecoratedLambda : Lambda {
using Out = bool;
};
return DecoratedLambda{std::forward<decltype(lambda)>(lambda)};
};
call(decorateLambda([&](auto&& value) {
(void)(value);
return true;
}));
}
With -O2 -std=c++20 -Wall:
<source>: In lambda function:
<source>:15:19: warning: typedef 'using
test()::<lambda(auto:1&&)>::DecoratedLambda::Out = bool' locally defined but
not used [-Wunused-local-typedefs]
15 | using Out = bool;
| ^~~
Compiler returned: 0
However, DecoratedLambda::Out *is* used by call(), both in the signature and in
the body. Perhaps the issue is that typedefs in local types that are returned
(or are reachable from returned types) shouldn't be considered "local"?
