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c++c++17c++20stdarraydeduction-guide

The deduction guide for std::array

In the C++ 17 and C++ 20 Working Drafts of the C++ Standard the deduction guide for the class template std::array is defined the following way

template<class T, class... U>
array(T, U...) -> array<T, 1 + sizeof...(U)>;

As a result for example this declaration

std::array a = { 1ll, 2llu };

should be compiled and the deduced type of the variable a is std::array<long long, 2>.

However compilers use another deduction guide that checks that all initializers have the same type.

Is it a bug of compilers or was the deduction guide indeed changed in C++ 17 and C++20 Standards?

Solution

  • C++17 has that requirement in the deduction guide.

    template<class T, class... U>
array(T, U...) -> array<T, 1 + sizeof...(U)>;

    Requires: (is_­same_­v<T, U> && ...) is true. Otherwise the program is ill-formed.

    [array.cons#2]