(You mean -> valarray<T>; not -> valarray;)

Yes, I think the described deduction guide is correct as written in the Standard and will work correctly. There is no requirement that a deduction guide has exactly the same parameter types as a constructor. Class template argument deduction deduction involves an overload resolution process, but that's entirely independent of the overload resolution process which then chooses a constructor of the deduced class type. After the specific class type is determined, it just needs to be valid to construct that class from the initializer in some way, following the usual rules. When the deduced class type is defined by a partial or explicit instantiation rather than the primary class template, the constructors used for initialization can be entirely different from the primary class constructors which were used in deducing the class template arguments.

In this case, if a placeholder class type is deduced by selecting the deduction guide
    template<class T, size_t cnt> valarray(const T(&)[cnt], size_t) -> valarray<T>;
then the constructor used will be
    valarray(const T*, size_t);
via the array-to-pointer conversion.

I think if the deduction guide were not specified, then code like
    const double a[] = {1.0, 1.2, 2.9};
    std::valarray va(a, sizeof(a)/sizeof(*a));
would use
    valarray(const T&, size_t)
to deduce T as const double[3] and get decltype(va) as std::valarray<const double[3]>, which won't work as likely intended, if at all.

But as it is, that code can use the deduction guide to deduce T as double, so decltype(va) is std::valarray<double>, and the initializer (a, sizeof(a)/sizeof(*a)) can initialize the va object using the valarray<double>::valarray(const double*, size_t) constructor.

On Tue, Oct 8, 2019 at 11:41 AM Vladimir Grigoriev via Std-Discussion <std-discussion@lists.isocpp.org> wrote:
For the class std::valarray<T> there is the following deduction guide

template<class T, size_t cnt> valarray(const T(&)[cnt], size_t) -> valarray;

However such a constructor is not listed among constructors of the class std::valaary<T>.

Is it correct?

With nest regards,
Vlad from Moscow

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