Created attachment 35697 [details] 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)
Reduced: template<typename... T> struct tuple { }; template<typename... T> tuple<T...> make_tuple(T&&...) { return {}; } template <typename... Params> void foo(Params... params) { auto t = make_tuple((int)params...); } template <typename... Params> void bar(Params... params) { tuple<decltype((int)params)...> t = make_tuple((int)params...); } int main() { foo(1,2,3); // Fails, clang++ compiles it bar(1,2,3); // Compiles correctly } t.cc: In instantiation of ‘void foo(Params ...) [with Params = {int, int, int}]’: t.cc:16:14: required from here t.cc:7:39: error: conversion from ‘tuple<int, int, int>’ to non-scalar type ‘tuple<int>’ requested auto t = make_tuple((int)params...); ^
Appears resolved in version r224163
Then let's add the testcase and close the bug.
Note that this is already fixed for 5.2.0 too.
Author: paolo Date: Wed Jul 8 16:08:10 2015 New Revision: 225563 URL: https://gcc.gnu.org/viewcvs?rev=225563&root=gcc&view=rev Log: 2015-07-08 Paolo Carlini <paolo.carlini@oracle.com> PR c++/66421 * g++.dg/cpp0x/auto45.C: New. Added: trunk/gcc/testsuite/g++.dg/cpp0x/auto45.C Modified: trunk/gcc/testsuite/ChangeLog
Done.
*** Bug 57565 has been marked as a duplicate of this bug. ***