I run into a problem with unpacking variadic templates into a template alias.
The following code works with Clang 3.4 and GCC 4.8 but fails with GCC 4.9:
template <typename T, typename...>
using front_type = T;
template <typename... Ts>
struct foo
{
using front = front_type<Ts...>;
};
GCC 4.9 complains:
test.cc:7:37: error: pack expansion argument for non-pack parameter 'T' of alias template 'template<class T, class ...> using front_type = T'
using front = front_type<Ts...>;
^
test.cc:1:15: note: declared here
template <typename T, typename...>
^
There exists a filed GCC bug (#59498), but is this supposed to fail? Here is some context from the C++ core language issue #1430, "pack expansion into fixed alias template parameter list":
Originally, a pack expansion could not expand into a fixed-length template parameter list, but this was changed in N2555. This works fine for most templates, but causes issues with alias templates.
In most cases, an alias template is transparent; when it's used in a template we can just substitute in the dependent template arguments. But this doesn't work if the template-id uses a pack expansion for non-variadic parameters. For example:
template<class T, class U, class V> struct S {}; template<class T, class V> using A = S<T, int, V>; template<class... Ts> void foo(A<Ts...>);
There is no way to express
A<Ts...>
in terms ofS
, so we need to hold onto theA
until we have theT
s to substitute in, and therefore it needs to be handled in mangling.Currently, EDG and Clang reject this testcase, complaining about too few template arguments for A. G++ did as well, but I thought that was a bug. However, on the ABI list John Spicer argued that it should be rejected.
It was reported in gcc4.9 bugzilla:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498
Minimal code to reproduce
template <typename T, typename ...>
using alias = T;
template <typename ...T>
using variadic_alias = alias<T...>;
using Fail = variadic_alias<int>;
int main() { }
From the explanation from gcc folks - it is not so obvious this is a real bug. This is still discussion held in gcc bugzilla and in DR 1430 (http://open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1430) - summary in now in the question above.
来源:https://stackoverflow.com/questions/24433658/unpacking-parameter-packs-in-template-aliases