可变模板折叠程序在gcc9中失败

Question

我在-std=c++14和-std=c++1z中使用g++ 8.3实现了如下程序的正常工作。但是，当我升级到g++ 9.1时，它只能在-std=c++14上工作，而不适用于更高的版本。

#include <iostream>
using namespace std;

#if __cplusplus >= 201500L // C++17 or higher

template <typename... Args>
auto sum(Args&&... args) 
{
    return (0 + args);// + ... );
}

#elif __cplusplus >= 201402L // C++14

auto sum() { return 0; }
template <typename T>
auto sum(T&& t) { return t; }
template <typename T, typename... Rest>
auto sum(T&& t, Rest&&... r) 
{
    return t + sum(forward<Rest>(r)...);
}

#else
#error "***You need C++14 or higher to compile this program***"
#endif

int main()
{
    cout << sum() << " result of 2 + 3=>" << sum(2, 3) <<
        " result of 12+15+18=>" << sum(12, 15, 18) << "\n";
#if __cplusplus >= 201500L // C++17 or higher
    cout << "Compiled as C++17 program\n";
#else
    cout << "Compiled as C++14 program\n";
#endif
}

g++ 9.1在使用-std=c++1z、c++17或c++2a时生成以下错误消息(与其兄弟姐妹一起)：

In function 'auto sum(Args&& ...)':
error: parameter packs not expanded with '...':
   12 |     return (0 + args);// + ... );
      |            ~~~^~~~~~~
note:         'args'
In function 'int main()':
error: no match for 'operator<<' (operand types are 'std::ostream' {aka 'std::basic_ostream<char>'} and 'void') 
   32 |     cout << sum() << " result of 2 + 3=>" << sum(2, 3) <<
      |     ~~~~ ^~ ~~~~~
      |     |          |   
      |     |          void
      |     std::ostream {aka std::basic_ostream<char>}

Answer 1

(0 + args)不是有效的折叠表达式。

你想要(0 + ... + args)。

另外，对于我来说，GCC 8.3 (使用-std=c++1z) "+<<+sum(2,+3)+<< ++++++++"+result+of+12+15+18=>"+<<+sum(12,+15,+18)+<<+"\n"; #if+__cplusplus+>=+201500L+//+C++17+or+higher ++++cout+<<+"Compiled+as+C++17+program\n"; #else ++++cout+<<+"Compiled+as+C++14+program\n"; #endif }'),l:'5',n:'0',o:'C+++source+#1',t:'0')),k:50,l:'4',n:'0',o:'',s:0,t:'0'),(g:!((g:!((h:compiler,i:(compiler:g83,filters:(b:'0',binary:'1',commentOnly:'0',demangle:'0',directives:'1',execute:'1',intel:'0',libraryCode:'1',trim:'1'),lang:c++,libs:!(),options:'-std=c++1z',source:1),l:'5',n:'0',o:'x86-64+gcc+8.3+(Editor+#1,+Compiler+#1)+C++',t:'0')),k:50,l:'4',m:50,n:'0',o:'',s:0,t:'0'),(g:!((h:output,i:(compiler:1,editor:1,wrap:'1'),l:'5',n:'0',o:'#1+with+x86-64+gcc+8.3',t:'0')),header:(),l:'4',m:50,n:'0',o:'',s:0,t:'0')),k:50,l:'3',n:'0',o:'',t:'0')),l:'2',n:'0',o:'',t:'0')),version:4">废品您的代码。