无法编译 std：：reduce 调用，而 std：：累积调用编译相同的参数

我一直在尝试使用c++17的std::reduce算法。据说，它应该支持std::accumulate支持的相同 API，但是当使用--std=c++17在clang++-9.0和gcc-9.2中编译时，对std::accumulate的调用编译成功，而对std::reduce的调用则不然。

到目前为止，我尝试了几件事：

• 定义调用内部和外部的lambda
• 使用std::reduce<...>语法显式指定类型
• 将重载与执行策略参数结合使用
  • #include <execution>出现错误

下面是一个示例代码，它使用std::reduce和std::accumulate对std::vector中的字符串长度求和：

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <numeric>
int main() {
auto op = [](auto acc, auto val) {
return acc + val.size();
};
std::vector<std::string> v = {"a", "bb", "ccc", "dddd"};
int a = std::reduce(v.begin(), v.end(), 0, op);
int b = std::accumulate(v.begin(), v.end(), 0, op);
return 0;
}

编译失败，在调用std::reduce的行中，clang-9.0.0的以下输出：

<source>:12:13: error: no matching function for call to 'reduce'
int a = std::reduce(v.begin(), v.end(), 0, op);
^~~~~~~~~~~
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/pstl/glue_numeric_defs.h:26:1: note: candidate template ignored: deduced conflicting types for parameter '_ForwardIterator' ('__gnu_cxx::__normal_iterator<std::__cxx11::basic_string<char> *, std::vector<std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> > > >' vs. 'int')
reduce(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Tp __init);
^
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/pstl/glue_numeric_defs.h:21:1: note: candidate function template not viable: requires 5 arguments, but 4 were provided
reduce(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Tp __init,
^
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/pstl/glue_numeric_defs.h:31:1: note: candidate function template not viable: requires 3 arguments, but 4 were provided
reduce(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last);
^
1 error generated.

以及来自gcc-9.2的以下输出：

<source>: In function 'int main()':
<source>:12:50: error: no matching function for call to 'reduce(std::vector<std::__cxx11::basic_string<char> >::iterator, std::vector<std::__cxx11::basic_string<char> >::iterator, int, main()::<lambda(auto:1, auto:2)>&)'
12 |     int a = std::reduce(v.begin(), v.end(), 0, op);
|                                                  ^
In file included from /opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/numeric:229,
from <source>:5:
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/pstl/glue_numeric_defs.h:21:1: note: candidate: 'template<class _ExecutionPolicy, class _ForwardIterator, class _Tp, class _BinaryOperation> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2> std::reduce(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Tp, _BinaryOperation)'
21 | reduce(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Tp __init,
| ^~~~~~
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/pstl/glue_numeric_defs.h:21:1: note:   template argument deduction/substitution failed:
<source>:12:50: note:   deduced conflicting types for parameter '_ForwardIterator' ('__gnu_cxx::__normal_iterator<std::__cxx11::basic_string<char>*, std::vector<std::__cxx11::basic_string<char> > >' and 'int')
12 |     int a = std::reduce(v.begin(), v.end(), 0, op);
|                                                  ^
In file included from /opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/numeric:229,
from <source>:5:
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/pstl/glue_numeric_defs.h:26:1: note: candidate: 'template<class _ExecutionPolicy, class _ForwardIterator, class _Tp> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2> std::reduce(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Tp)'
26 | reduce(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Tp __init);
| ^~~~~~
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/pstl/glue_numeric_defs.h:26:1: note:   template argument deduction/substitution failed:
<source>:12:50: note:   deduced conflicting types for parameter '_ForwardIterator' ('__gnu_cxx::__normal_iterator<std::__cxx11::basic_string<char>*, std::vector<std::__cxx11::basic_string<char> > >' and 'int')
12 |     int a = std::reduce(v.begin(), v.end(), 0, op);
|                                                  ^
In file included from /opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/numeric:229,
from <source>:5:
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/pstl/glue_numeric_defs.h:31:1: note: candidate: 'template<class _ExecutionPolicy, class _ForwardIterator> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, typename std::iterator_traits<_II>::value_type> std::reduce(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator)'
31 | reduce(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last);
| ^~~~~~
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/pstl/glue_numeric_defs.h:31:1: note:   template argument deduction/substitution failed:
<source>:12:50: note:   deduced conflicting types for parameter '_ForwardIterator' ('__gnu_cxx::__normal_iterator<std::__cxx11::basic_string<char>*, std::vector<std::__cxx11::basic_string<char> > >' and 'int')
12 |     int a = std::reduce(v.begin(), v.end(), 0, op);
|                                                  ^