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