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1 // PR c++/66937
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2 // { dg-do compile { target c++17_only } }
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3 // { dg-options "-fconcepts" }
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4
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5 #include <tuple>
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6
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7 namespace detail
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8 {
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9 template<typename T, template<typename...> class Sink>
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10 struct copy_tuple_args_impl;
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11
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12 template<typename... Args, template<typename...> class Sink>
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13 struct copy_tuple_args_impl<std::tuple<Args...>, Sink>
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14 {
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15 using type = Sink<Args...>;
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16 };
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17 }
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18
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19 // copy_tuple_args copies the template arguments of a tuple into another template
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20 // copy_tuple_args does not care about constraints whatsoever.
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21 template<typename Tuple, template<typename...> class Sink>
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22 using copy_tuple_args = typename detail::copy_tuple_args_impl<Tuple, Sink>::type;
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23
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24 // A concept of a column
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25 template <typename T>
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26 concept bool Column()
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27 {
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28 return requires()
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29 {
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30 typename T::_name_t;
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31 };
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32 }
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33
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34 // column_list is constrained to Column arguments
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35 template<Column... C>
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36 struct column_list
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37 {
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38 };
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39
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40 // Here are some columns
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41 struct A
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42 {
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43 using _name_t = int;
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44 };
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45
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46 struct B
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47 {
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48 using _name_t = int;
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49 };
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50
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51
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52 int main()
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53 {
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54 using ColumnTuple = std::tuple<A, B>;
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55 using ColumnList = copy_tuple_args<ColumnTuple, column_list>; // This fails, but should not
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56 }
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