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1 // Testcase from P0170R1
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2 // { dg-do compile { target c++17 } }
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3
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4 auto monoid = [](auto v) { return [=] { return v; }; };
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5 auto add = [](auto m1) constexpr {
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6 auto ret = m1();
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7 return [=](auto m2) mutable {
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8 auto m1val = m1();
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9 auto plus = [=] (auto m2val) mutable constexpr
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10 { return m1val += m2val; };
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11 ret = plus(m2());
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12 return monoid(ret);
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13 };
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14 };
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15
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16 int main()
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17 {
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18 constexpr auto zero = monoid(0);
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19 constexpr auto one = monoid(1);
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20 static_assert(add(one)(zero)() == one()); // OK
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21 // Since 'two' below is not declared constexpr, an evaluation of its constexpr
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22 // member function call operator can not perform an lvalue-to-rvalue conversion
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23 // on one of its subobjects (that represents its capture) in a constant
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24 // expression.
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25 auto two = monoid(2);
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26 if (!(two() == 2)) __builtin_abort(); // OK, not a constant expression.
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27 static_assert(add(one)(one)() == two()); // { dg-error "|in .constexpr. expansion of " } two() is not a constant expression
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28 static_assert(add(one)(one)() == monoid(2)()); // OK
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29 }
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