How can std::ranges work with initializer_list?

Question

I wonder if the std::ranges::* algorithms should be working with std::initializer_list. For example, line (B) in the following snippet compiles while line (A) does not, when compiling with x64 MSVC v19.latest and option /std:c++latest (godbolt.org/z/5oTEz1q5P).

#include <algorithm>
#include <array>
#define ONE 1
#define TWO 2
static_assert(std::ranges::is_sorted({ ONE, TWO }));  // (A)
static_assert(std::ranges::is_sorted(std::array { ONE, TWO }));  // (B)
int main() {}

The underlying temporary array that is allocated for the initializer_list should live until the call to is_sorted() in line (A) has returned.

According to the error message, std::initializer_list cannot be casted to a std::ranges::forward_range: algorithm(10432): 'bool std::ranges::_Is_sorted_fn::operator ()(_Rng &&,_Pr,_Pj) const': could not deduce template argument for '_Rng'.

Is this the same reason as this one?

Answer 1

The problem with your code std::ranges::is_sorted({ ONE, TWO }), is that it cannot deduce the type of { ONE, TWO } within a template parameter.

To actually do this, you must either manually create a initializer_list first:

std::ranges::is_sorted(std::initializer_list{ ONE, TWO })

Or the function itself must not deduce the type of this parameter.

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Sidenote, this is one place where the behavior of auto differ from template deduction.

auto is specifically given the ability to deduce il within this syntax:

auto il = {a, b, c};

No other usually equivalent syntaxes can deduce this as initializer_list, including auto il{a, b, c}, template_func({a, b, c}), and etc..