[Bug c++/64588] New: [C++11] Wrong number of template arguments when template template parameter is template alias.

["splinterofchaos at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64588

            Bug ID: 64588
           Summary: [C++11] Wrong number of template arguments when
                    template template parameter is template alias.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: splinterofchaos at gmail dot com

As far as I have been able to gather, and according to everyone I've asked, the
following should be valid:


#include <utility>

template<class T>
using decay = typename std::decay<T>::type;

template<template<class...>class T, class...X>
struct Part {
  template<class...Y>
  using type = T<X..., Y...>;
};

template<template<class...>class T, template<class>class U>
struct UCompose {
  // Not ok!
  template<class X, class...Y>
  using type = T<U<X>, Y...>;

  //// Ok!
  //template<class X>
  //using type = T<U<X>>;
};

int main() {
  using IsInt = Part<std::is_same, int>;
  static_assert(IsInt::type<int>::value, "");    // Ok!
  using IsIntD = UCompose<IsInt::type, decay>;
  static_assert(IsIntD::type<int&>::value, "");  // Not ok.
}


The error is:


$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.9/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.9.1-16ubuntu6'
--with-bugurl=file:///usr/share/doc/gcc-4.9/README.Bugs
--enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.9 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.9 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-vtable-verify
--enable-plugin --with-system-zlib --disable-browser-plugin
--enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.9-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.9.1 (Ubuntu 4.9.1-16ubuntu6) 

$ g++ tmp.cpp --std=c++11 -Wall -Wextra -save-temps
tmp.cpp: In substitution of âtemplate<template<class ...> class T, class ... X>
template<class ... Y> using type = T<X ..., Y ...> [with Y = {typename
std::decay<_Tp>::type, Y ...}; T = std::is_same; X = {int}]â:
tmp.cpp:16:29:   required from âstruct UCompose<Part<std::is_same, int>::type,
decay>â
tmp.cpp:27:23:   required from here
tmp.cpp:9:29: error: wrong number of template arguments (3, should be 2)
   using type = T<X..., Y...>;
                             ^
In file included from /usr/include/c++/4.9/bits/move.h:57:0,
                 from /usr/include/c++/4.9/bits/stl_pair.h:59,
                 from /usr/include/c++/4.9/utility:70,
                 from tmp.cpp:1:
/usr/include/c++/4.9/type_traits:922:12: error: provided for âtemplate<class,
class> struct std::is_sameâ
     struct is_same;
            ^


The message, "wrong number of template arguments" implies that three parameters
are passed, but above it says "X = {int}" and "Y = {typename
std::decay<_Tp>::type, Y ...}". The "Y ..." here should be empty; perhaps it
comes from "UCompose::type", where again the "Y..." should be empty.

I have tried with gcc 4.8 and get the same error message. Attached is the
output from -save-temps