#include <iostream>
#include <functional>
 
template<typename... Args>
void testfunc(const std::function<void (float,Args...,char)>& func)
{
 
}
 
int main(int argc,char* argv[])
{
    auto func = [](float,int,char) {};
    auto sfunc = static_cast<std::function<void (float,char)>>(func);
    testfunc<int>(sfunc);
 
    return 0;
}

我明确指定了类型,因为(https://stackoverflow.com/a/40476083)：

When a parameter pack doesn’t appear last in the parameter
declaration,it is a non-deduced context. A non-deduced context means
that the template arguments have to be given explicitly.

MSVC成功编译它,而gcc和clang都拒绝代码：

source_file.cpp: In function ‘int main(int,char**)’:
source_file.cpp:14:24: error: no matching function for call to ‘testfunc(std::function<void(float,char)>&)’
     testfunc<int>(sfunc);
                        ^
source_file.cpp:5:6: note: candidate: template<class ... Args> void testfunc(const std::function<void(float,Args ...,char)>&)
 void testfunc(const std::function<void (float,char)>& func)
      ^
source_file.cpp:5:6: note:   template argument deduction/substitution Failed:
source_file.cpp:14:24: note:   mismatched types ‘char’ and ‘int’
     testfunc<int>(sfunc);
                        ^
source_file.cpp:14:24: note:   ‘std::function<void(float,char)>’ is not derived from ‘const std::function<void(float,char)>’

现在让我们稍作改动 – 让我们从本地func中删除int参数,从而导致模板参数包变空：

#include <iostream>
#include <functional>
 
template<typename... Args>
void testfunc(const std::function<void (float,char)>>(func);
    testfunc<>(sfunc);
 
    return 0;
}

这一次,所有三个编译器都拒绝代码不正确.
使用http://rextester.com/l/cpp_online_compiler_gcc和本地Visual Studio安装进行测试.

问题：

>第一种情况谁是正确的？
>如何实现预期效果 – 即,如何明确指定(可能为空)参数包？