Visual Studio 2017-无法推断模板参数(带有可变参数模板) [英] Visual Studio 2017 - could not deduce template argument (with variadic templates)

问题描述

以下代码可在gcc和clang中编译并正常工作，但无法在Visual Studio 2017.7(x86-64)中编译:

The following code compiles and works fine in gcc and clang, but fails to compile in Visual Studio 2017.7 (x86-64):

#include <vector>
#include <iostream>
#include <type_traits>


template <template <typename...> class>
struct isVector : public std::false_type
{ };

template <>
struct isVector<std::vector> : public std::true_type
{ };

// Other isVector specializations (for QVector<T>, etc...)
// ...


// A function accepting vector<vector<double>>
template <
    template<typename ...> class V1,
    template<typename ...> class V2 >
auto loadData(V1<V2<double>> & v, std::string fn)
-> std::enable_if_t<isVector<V1>::value && isVector<V2>::value>
{
    std::cout << "- vector of vector version, " << fn << std::endl;
}

// A function accepting vector<double>
template <
    template<typename ...> class V >
auto loadData(V<double> & v, std::string fn)
-> std::enable_if_t<isVector<V>::value>
{
    std::cout << "- vector version, " << fn << std::endl;

    V<V<double>> vv({ v });

    loadData(vv, fn);
}

// Other loadData() function specializations and overloads
// ...


int main()
{
    std::vector<double> vd;
    std::vector<std::vector<double>> vvd;

    loadData(vd, "case  1");
    loadData(vvd, "case  2");

    return 0;
}

这是错误消息:

<source>(50): error C2672: 'loadData': no matching overloaded function found
<source>(50): error C2784: 'enable_if<_Test,_Ty>::type loadData(V<double> &,std::string)': could not deduce template argument for 'V<double> &' from 'std::vector<double,std::allocator<_Ty>>'
        with
        [
            _Ty=void
        ]
        and
        [
            _Ty=double
        ]
<source>(31): note: see declaration of 'loadData'
<source>(50): error C2782: 'enable_if<_Test,_Ty>::type loadData(V<double> &,std::string)': template parameter 'V' is ambiguous
        with
        [
            _Ty=void
        ]
<source>(31): note: see declaration of 'loadData'
<source>(50): note: could be 'std::_Vector_alloc'
<source>(50): note: or       'std::vector'
<source>(50): error C2784: 'enable_if<_Test,_Ty>::type loadData(V1<V2<double>> &,std::string)': could not deduce template argument for 'V1<V2<double>> &' from 'std::vector<double,std::allocator<_Ty>>'
        with
        [
            _Ty=void
        ]
        and
        [
            _Ty=double
        ]
<source>(22): note: see declaration of 'loadData'
<source>(50): error C2782: 'enable_if<_Test,_Ty>::type loadData(V1<V2<double>> &,std::string)': template parameter 'V1' is ambiguous
        with
        [
            _Ty=void
        ]
<source>(22): note: see declaration of 'loadData'
<source>(50): note: could be 'std::_Vector_alloc'
<source>(50): note: or       'std::vector'
<source>(51): error C2672: 'loadData': no matching overloaded function found
<source>(51): error C2784: 'enable_if<_Test,_Ty>::type loadData(V<double> &,std::string)': could not deduce template argument for 'V<double> &' from 'std::vector<std::vector<double,std::allocator<_Ty>>,std::allocator<std::vector<_Ty,std::allocator<_Ty>>>>'
        with
        [
            _Ty=void
        ]
        and
        [
            _Ty=double
        ]
<source>(31): note: see declaration of 'loadData'
<source>(51): error C2782: 'enable_if<_Test,_Ty>::type loadData(V<double> &,std::string)': template parameter 'V' is ambiguous
        with
        [
            _Ty=void
        ]
<source>(31): note: see declaration of 'loadData'
<source>(51): note: could be 'std::_Vector_alloc'
<source>(51): note: or       'std::vector'
<source>(51): error C2784: 'enable_if<_Test,_Ty>::type loadData(V1<V2<double>> &,std::string)': could not deduce template argument for 'V1<V2<double>> &' from 'std::vector<std::vector<double,std::allocator<_Ty>>,std::allocator<std::vector<_Ty,std::allocator<_Ty>>>>'
        with
        [
            _Ty=void
        ]
        and
        [
            _Ty=double
        ]
<source>(22): note: see declaration of 'loadData'
<source>(51): error C2782: 'enable_if<_Test,_Ty>::type loadData(V1<V2<double>> &,std::string)': template parameter 'V2' is ambiguous
        with
        [
            _Ty=void
        ]
<source>(22): note: see declaration of 'loadData'
<source>(51): note: could be 'std::_Vector_alloc'
<source>(51): note: or       'std::vector'

这是VS还是代码错误?关于如何解决它的任何建议?我确实尝试过使用/permissive-和/std:latest 进行编译，没有任何区别.

Is this a VS or a code bug? Any suggestions on how to fix it? I did try compiling with /permissive- and /std:latest, didn't make a difference.

此处接受的答案中建议了原始代码:同时接受std :: vector和QVector的功能模板吗?

The original code was suggested in the accepted answer here: A function template that accepts both std::vector and QVector?

编译器资源管理器链接: https://godbolt.org/g/by7nBM

Compiler explorer link: https://godbolt.org/g/by7nBM

谢谢！

推荐答案

我也遇到了这个讨厌的小错误.我认为这也是一个错误.以下代码曾经在以前的版本下运行:

I too have bumped into this nasty little bug. I think it's a bug as well. The following code used to work under previous versions:

namespace Catch {
    template<typename Ty_, typename... Args_>
    std::string Tags(Ty_ x, Args_... others) {
        return Tags(x) + Tags(others...);
    }
}

依此类推，将很有用，如下所示:

And so on, and would be useful as follows:

TEST_CASE("my test case", Catch::Tags("one", "two", "fred", "red")) {
    // ...
}

使用 Catch2 ，不确定是否需要添加 .c_str()；至少在 Catch 中，我认为这是必要的.也许不再那么重要了.

With Catch2, not sure if you need to append an .c_str() on that; at least in Catch, I think that was necessary. Perhaps not so much any more.

我这样做是为了避免陷入标签语法的麻烦，我可以只关注标签本身，而保留可变参数函数的样板语法.

I did this in an effort to keep from having to trip up over tag syntax, and I could simply focus on the tags themselves and leave the boilerplate syntax for the variadic functions.

但是...现在我已经犯了这个编译器错误.

However... Now I've run afoul of this compiler bug.

任何想法，如果有 查看全文