gcc warning "-WOverflow" - c++

Question

please consider the following snippet of C++98 code:
#include <iostream>
#include <limits>
template <typename T> T myLower() {
if (std::numeric_limits<T>::is_integer) {
return std::numeric_limits<T>::min();
} else {
return -std::numeric_limits<T>::max();
}
}
int main(int argc, char *argv[]) {
std::cout << (int)myLower<uint8_t>() << "\n";
return 0;
}
When compiled with g++ test.cpp && ./a.out it works as expected: 0 is printed. But when compiled with any optimization (-O1 upwards, -Os as well) the -Woverflow warning is emitted for the else-branch, which should only be there for floating-point numbers:
g++ test.cpp -Wall -O1 && ./a.out
test.cpp: In instantiation of ‘T myLower() [with T = unsigned char]’:
test.cpp:16:46: required from here
test.cpp:9:16: warning: large integer implicitly truncated to unsigned type [-Woverflow]
retval = -std::numeric_limits<T>::max();
~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
0
This also fails for uint16, all other types work as they should. As far as I can see the std::numeric_limits<uint8_t>::is_integer should be a compile-time constant -- why is the else-branch compiled at all? The warning is not serious or did I miss something?
Ah, this happens in gcc5 and gcc6.
Thanks!
PS: I know that there will be std::numeric_limits<>::lowest() when the future will have arrived here...

Answer 1

The problem here is that
if (std::numeric_limits<T>::is_integer)
Does not limit what T can be. That means the else brance is always compiled and can trigger a warning if the operation would cause an overflow even if the branch is never reached.
If you had C++17 you could use if constexpr like
template <typename T> T myLower() {
if constexpr (std::numeric_limits<T>::is_integer) {
return std::numeric_limits<T>::min();
} else {
return -std::numeric_limits<T>::max();
}
}
Which would only compile the if block or the else block depending on the constexpr condition. Before C++17 you would have to use SFINAE to constrain the function to only compile if the template resolves.