https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90881
Bug ID: 90881
Summary: -Wunused-value false positive with
Product: gcc
Version: 9.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: federico.kircheis at gmail dot com
Target Milestone: ---
Hello,
consider
----
#include <type_traits>
template <typename T, typename = void> struct status : std::false_type{};
template <typename T> struct status<T, decltype(T::member, void())> :
std::true_type {};
struct s1{int member;};
struct s2{int _member;};
int main(){
static_assert(status<s1>::value, "has member");
static_assert(!status<s2>::value, "has no member");
}
----
When compiling with `-Wall` (or `-Wunused-value`), g++ generates following
warning:
--
bug.cpp: In instantiation of ‘struct status<s1>’:
bug.cpp:11:26: required from here
bug.cpp:5:58: warning: left operand of comma operator has no effect
[-Wunused-value]
template <typename T> struct status<T, decltype(T::member, void())> :
std::true_type {};
~~~~~~~~~^~~~~~~~
bug.cpp:5:58: warning: left operand of comma operator has no effect
[-Wunused-value]
--
The warning is incorrect, as removing the left operand changes the behaviour of
the program, thus it has effect:
----
#include <type_traits>
template <typename T, typename = void> struct status : std::false_type{};
template <typename T> struct status<T, decltype(void())> : std::true_type {};
// ! removed left operand
struct s1{int member;};
struct s2{int _member;};
int main(){
static_assert(status<s1>::value, "has member");
static_assert(!status<s2>::value, "has no member");
}
----
Does not compile:
--
bug.cpp: In function ‘int main()’:
bug.cpp:12:16: error: static assertion failed: has no member
static_assert(!status<s2>::value, "has no member");
--
because `status` does not detect anymore if the member-variable `member` is
present or not.
