为什么可变参数模板在模板引入中不起作用，而在需求子句中起作用？ ConceptName {T，U，V，W} &lt;—模板 <typename …T>

Question

We have:

template <typename ...T> concept bool Numerics = ( std::is_arithmetic_v<T> && ... ) ;
template <typename T>    concept bool Numeric  =   std::is_arithmetic_v<T>;

So we can apply type constraints with a requires-clause like this:

template <typename T, typename U, typename V, typename W> requires Numerics<T,U,V,W>
auto foo(T arg1, U arg2, V arg3, W arg4) {
    return arg1 + arg2 + arg3 + arg4;
}

But we can not write in template introduction format like this:

// err: no match concept
// 
// Numerics{T,U,V,W}
// auto foo2(T arg1, U arg2, V arg3, W arg4) {
//     return arg1 + arg2 + arg3 + arg4;
// }

Having to explicitly define fixed number of arguments instead:

template <typename T, typename U, typename V, typename W>
                         concept bool Numeric4 =   Numerics<T,U,V,W>;

Numeric4{T,U,V,W}
auto foo3(T arg1, U arg2, V arg3, W arg4) {
    return arg1 + arg2 + arg3 + arg4;
}

Why doesn't template <typename ...T> concept work in template introduction format while work in requires-clause?

LIVE

Answer 1

First, note that this syntax has been removed from the Concepts TS in its latest draft .

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In the previous draft , this syntax was defined in [temp.intro] and is well-defined:

Numerics{T,U,V,W}
auto foo2(T arg1, U arg2, V arg3, W arg4) {
    return arg1 + arg2 + arg3 + arg4;
}

should, for each introduced-parameter , adjust the parameter pack in Numerics by its pattern, and declare a template parameter based on that pattern. So this should be equivalent to:

template <typename T, typename U, typename V, typename W> // per [temp.intro]/2
     requires Numerics<T,U,V,W> // per [temp.intro]/5
auto foo2(T arg1, U arg2, V arg3, W arg4) {
    return arg1 + arg2 + arg3 + arg4;
}

There are further examples in this section which illustrate that this should work. Per this draft, the code is well-formed.

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That said, as previously noted, the syntax has been removed from the TS and does not appear in the C++20 working draft. It may or may not be added in the future, in this form or otherwise.