[Bug c++/66421] New: G++ fails compilation when assigning tuple created with variadic template to auto variable

[svalorzen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66421

            Bug ID: 66421
           Summary: G++ fails compilation when assigning tuple created
                    with variadic template to auto variable
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: svalorzen at gmail dot com
  Target Milestone: ---

Created attachment 35697
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35697&action=edit
File generated with -save-temps

The following code contains two equivalent functions, one declaring a variable
with auto and the other specifying the variable type explicitly. The function
using auto fails to compile, although the two functions provided are
equivalent. clang++ successfully compiles the code:

    #include <tuple>

    template <size_t N, typename T, typename... Args>
    struct DimTupleImpl {
        using type = typename DimTupleImpl<N-1, T, Args..., T>::type;
    };

    template <typename T, typename... Args>
    struct DimTupleImpl<0, T, Args...> {
        using type = std::tuple<Args...>;
    };

    // Tuple with N elements all of type T
    template <size_t N, typename T>
    struct DimTuple {
        using type = typename DimTupleImpl<N, T>::type;
    };

    template <typename... Params>
    void foo(Params... params) {
        auto tuple = std::make_tuple((size_t)params...);
    }

    template <typename... Params>
    void bar(Params... params) {
        typename DimTuple<sizeof...(Params), size_t>::type tuple =
std::make_tuple((size_t)params...);
    }

    int main() {
        foo(1,2,3); // Fails, clang++ compiles it
        bar(1,2,3); // Compiles correctly
        return 0;
    }

# Compiled as: 

g++ -std=c++11 main.cpp

# Compiler output:

main.cpp: In instantiation of ‘void foo(Params ...) [with Params = {int, int,
int}]’:
main.cpp:30:14:   required from here
main.cpp:21:51: error: conversion from ‘std::tuple<long unsigned int, long
unsigned int, long unsigned int>’ to non-scalar type ‘std::tuple<long unsigned
int>’ requested
     auto tuple = std::make_tuple((size_t)params...);
                                                   ^
# System:

Lubuntu 15.04

# Compiler info:

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
4.9.2-10ubuntu13' --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 4.9.2 (Ubuntu 4.9.2-10ubuntu13)