http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58093
Bug ID: 58093
Summary: Semi-bogus warning about narrowing conversions and
variadic templates
Product: gcc
Version: 4.8.1
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: nmm1 at cam dot ac.uk
This may be related to 45397 and 53661, but it's not clear. The diagnostic
is both confusing and makes many uses of variadic templates extremely
confusing to their users.
4.13 [conv.rank], 4.7 [conv.integral] and 5 [expr] make it pretty clear
that this is not a narrowing conversion, which makes the diagnostic very
confusing. I agree that it potentially changes value, but the wording
could be better (e.g. 'loss of sign'). That's the trivial part of this
report.
The more serious (but still minor) one is that I don't think that this
is a soluble problem for the variadic template writer. It is unreasonable
to have to convert arguments explicitly, when it's automatic under almost
all circumstances.
The following program:
#include <iostream>
using namespace std;
template<typename ... Pack>
void weeble (Pack ... Dims) {
size_t r[] = {Dims...};
for (int i = 0; i < sizeof...(Dims); ++i)
std::cout << " " << r[i];
std::cout << std::endl;
}
int main() {
weeble(7,13,42,69);
}
produces:
junk.cpp: In instantiation of 'void weeble(Pack ...) [with Pack = {int, int,
int, int}]':
junk.cpp:13:22: required from here
junk.cpp:6:26: warning: narrowing conversion of 'Dims#0' from 'int' to 'size_t
{aka long unsigned int}' inside { } [-Wnarrowing]
size_t r[] = {Dims...};
^
junk.cpp:6:26: warning: narrowing conversion of 'Dims#1' from 'int' to 'size_t
{aka long unsigned int}' inside { } [-Wnarrowing]
junk.cpp:6:26: warning: narrowing conversion of 'Dims#2' from 'int' to 'size_t
{aka long unsigned int}' inside { } [-Wnarrowing]
junk.cpp:6:26: warning: narrowing conversion of 'Dims#3' from 'int' to 'size_t
{aka long unsigned int}' inside { } [-Wnarrowing]
g++ -v produces:
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/home/nmm/GCC/lib/gcc/x86_64-unknown-linux-gnu/4.8.1/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --prefix=/home/nmm/GCC --disable-bootstrap
--enable-languages=c,c++,fortran --enable-werror=yes --disable-decimal-float
Thread model: posix
gcc version 4.8.1 (GCC)
