https://llvm.org/bugs/show_bug.cgi?id=24058
Bug ID: 24058
Summary: Suspicious warning on non-deducible parameter pack
Product: clang
Version: trunk
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: P
Component: C++11
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
Classification: Unclassified
The following snippet generates a suspicious looking warning:
#include <cstddef>
#include <tuple>
#include <utility>
template <typename ...Ts>
struct pack {};
template <typename LowIs, typename ...Ts>
struct _split;
template <std::size_t ...LowIs, typename ...Ts>
struct _split<std::index_sequence<LowIs...>, Ts...> {
using left = pack<
typename std::tuple_element<LowIs, std::tuple<Ts...>>::type...>;
template <typename Is>
struct _right;
template <std::size_t ...HighIs>
struct _right<std::index_sequence<LowIs..., HighIs...>> {
using type = pack<
typename std::tuple_element<HighIs,
std::tuple<Ts...>>::type...>;
};
using right = typename
_right<std::make_index_sequence<sizeof...(Ts)>>::type;
};
template <typename ...Ts>
struct split
: _split<std::make_index_sequence<sizeof...(Ts)/2>, Ts...>
{};
int main() {
split<int, float, double, bool>::left l = pack<int, float>{};
split<int, float, double, bool>::right r = pack<double, bool>{};
(void)l; (void)r;
}
The warning is:
prog.cc:20:12: warning: class template partial specialization contains a
template parameter that cannot be deduced; this partial specialization will
never be used
struct _right<std::index_sequence<LowIs..., HighIs...>> {
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
prog.cc:19:30: note: non-deducible template parameter 'HighIs'
template <std::size_t ...HighIs>
Regardless of it compilation succeeds, so the parameter is in fact being
deduced and the specialization used. From a cursory looking at the standard,
[temp.deduct.type]/9 seems to suggest the warning is correct and the snippet
should fail to compile.
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