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    <pre class="prettyprint" id="lang-cpp">
    #include &lt;type_traits&gt;

    template&lt;class T&gt;
    concept aggregate = std::is_aggregate_v&lt;std::remove_cvref_t&lt;T&gt;&gt;;
    
    // empty class convertible to anything
    struct ToAnything{ template&lt;class To&gt; operator To&&(); };

    // for an aggregate type 'T',
    // return the number of data members 
    template&lt;aggregate T&gt;
    consteval auto arity(auto... to_anything)
    {
        //  could 'T' be construct with one more argument ?
        if constexpr(requires{ T{ {to_anything}..., {ToAnything{}} }; })

            // yes ? keep trying with more argument
            return arity&lt;T&gt;(to_anything..., ToAnything{});

        // no ? so we found the aggregate arity
        // as we can aggregate initialize 'T' with {to_anything...} at most
        else
            return sizeof...(to_anything);
    }


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