7 Expressions [expr]

7.6 Compound expressions [expr.compound]

7.6.17 Yielding a value [expr.yield]

yield-expression:
co_yield assignment-expression
co_yield braced-init-list
A yield-expression shall appear only within a suspension context of a function ([expr.await]).
Let e be the operand of the yield-expression and p be an lvalue naming the promise object of the enclosing coroutine ([dcl.fct.def.coroutine]), then the yield-expression is equivalent to the expression co_­await p.yield_­value(e).
[Example 1: template <typename T> struct my_generator { struct promise_type { T current_value; /* ... */ auto yield_value(T v) { current_value = std::move(v); return std::suspend_always{}; } }; struct iterator { /* ... */ }; iterator begin(); iterator end(); }; my_generator<pair<int,int>> g1() { for (int i = i; i < 10; ++i) co_yield {i,i}; } my_generator<pair<int,int>> g2() { for (int i = i; i < 10; ++i) co_yield make_pair(i,i); } auto f(int x = co_yield 5); // error: yield-expression outside of function suspension context int a[] = { co_yield 1 }; // error: yield-expression outside of function suspension context int main() { auto r1 = g1(); auto r2 = g2(); assert(std::equal(r1.begin(), r1.end(), r2.begin(), r2.end())); } — end example]