https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63723
Bug ID: 63723
Summary: Narrowing conversion allowed in braced init list in
SFINAE context
Product: gcc
Version: 4.9.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: tavianator at gmail dot com
Created attachment 33877
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33877&action=edit
Preprocessed source
GCC allows narrowing conversions in braced init lists with only a warning. The
documentation suggests that this extension doesn't apply in SFINAE contexts,
but it appears to be wrong:
$ cat foo.cpp
#include <type_traits>
#include <utility>
template <typename From, typename To>
class is_list_convertible_helper
{
template <typename To2>
static void requires_conversion(To2 t);
template <typename From2, typename To2,
typename = decltype(requires_conversion<To2>({std::declval<From2>()}))>
// ^ Braced initializer
static std::true_type helper(int);
template <typename From2, typename To2>
static std::false_type helper(...);
public:
using type = decltype(helper<From, To>(0));
};
template <typename From, typename To>
class is_list_convertible
: public is_list_convertible_helper<From, To>::type
{ };
static_assert(!is_list_convertible<double, int>::value,
"double -> int is narrowing!");
$ g++ -std=c++11 foo.cpp
foo.cpp: In substitution of ‘template<class From2, class To2, class> static
std::true_type is_list_convertible_helper<From, To>::helper(int) [with From2 =
double; To2 = int; <template-parameter-1-3> = <missing>]’:
foo.cpp:18:31: required from ‘class is_list_convertible_helper<double, int>’
foo.cpp:22:7: required from ‘class is_list_convertible<double, int>’
foo.cpp:26:48: required from here
foo.cpp:10:46: warning: narrowing conversion of ‘std::declval<double>()’ from
‘double’ to ‘int’ inside { } [-Wnarrowing]
typename = decltype(requires_conversion<To2>({std::declval<From2>()}))>
^
foo.cpp:26:1: error: static assertion failed: double -> int is narrowing!
static_assert(!is_list_convertible<double, int>::value,
^
