Consider the following program:
template <unsigned int I>
int f(int x)
{
auto task = [&]() { ++x; };
if constexpr (I == 0) {
task();
}
return x;
}
int main()
{
f<1>(3);
}
Compiling on gcc 9.3 with -std=c++17 -Wall -pedantic
, it issues a warning
warning: variable 'task' set but not used [-Wunused-but-set-variable]
4 | auto task = [&]() { ++x; };
But with a newer gcc version, no such warning appears. Notice that according to the manual , -Wunused-but-set-variable
is enabled by -Wall
.
Also with clang, no such a warning appears.
Test it on godbolt .
Is that a compiler shortcoming, or is this behavior (the lack of warning) wanted/expected?
Consider this slightly modifed code:
template <unsigned int I>
int f(int x)
{
auto task = [&]() { ++x; };
if constexpr (I == 0) {
task();
}
return x;
}
int main()
{
f<0>(3);
f<1>(3);
}
With gcc 9.3 -std=c++2a -Wall -Werror
you get an error (warning treated as error) :
<source>: In instantiation of 'int f(int) [with unsigned int I = 1]':
<source>:14:11: required from here
<source>:4:10: error: variable 'task' set but not used [-Werror=unused-but-set-variable]
4 | auto task = [&]() { ++x; };
| ^~~~
cc1plus: all warnings being treated as errors
This is bad, because the code is completely fine and it can be considered a bug that it triggers a warning. Apparently this has been fixed in gcc >= 10.1.
One could argue that one could/should move declaration of task
into the if constexpr
branch, but then consider that this made gcc issue a warning as well:
template <unsigned int I>
int f(int x)
{
auto task = [&]() { ++x; };
if constexpr (I == 0) {
task();
} else if constexpr (I == 1) {
task();
}
return x;
}
int main()
{
f<0>(3);
f<1>(3);
f<2>(3);
}
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