exec::repeat_effect_until: Throwing Decay-Copy & Value Category Preservation #1723
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…rvation The asynchronous loop of exec::repeat_effect_until proceeds until the child operation sends a value which converts to true. Previously this process proceeded as follows: 1. Accept the child operation's result by value to avoid dangling references into the operation state (see next step) 2. Destroy the child operation state 3. Convert the result accepted in step 1 to bool and check if it's true, if so end the operation, otherwise 4. Connect the child sender 5. Start the new child operation Unfortunately step 1 meant that the result of the child operation would be decay-copied to pass it by value. This occurred within a noexcept function and therefore if that decay-copy threw std::terminate would be called. Moreover the previous implementation did not forward the result in step 3. This meant that if the child's result type was only rvalue convertible to bool compilation would fail. Additionally the same pass-by-value strategy was used for errors. However when handling an error there's no need to destroy the child operation state due to the fact the operation is ending and therefore doesn't need to reconnect the child sender for the next iteration (note this logic also applies to successful completion). Fixed all of the above by handling completion of the child operation as follows: 1. If the child completed with error or stopped simply forward that completion through (note the child operation state is not destroyed and will be cleaned up by the destructor of the operation state for exec::repeat_effect_until) (note that this saves one decay-copy over the previous implementation but requires a branch in the destructor, which was already present), otherwise 2. Forward the result, convert that forwarded expression to bool, and check if it's true, if so end the operation (note that once again the child operation state is not destroyed and once again a decay-copy is eliminated), otherwise 3. Destroy the child operation state 4. Connect the child sender 5. Start the new child operation
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The asynchronous loop of exec::repeat_effect_until proceeds until the child operation sends a value which converts to true. Previously this process proceeded as follows:
Unfortunately step 1 meant that the result of the child operation would be decay-copied to pass it by value. This occurred within a noexcept function and therefore if that decay-copy threw std::terminate would be called.
Moreover the previous implementation did not forward the result in step 3. This meant that if the child's result type was only rvalue convertible to bool compilation would fail.
Additionally the same pass-by-value strategy was used for errors. However when handling an error there's no need to destroy the child operation state due to the fact the operation is ending and therefore doesn't need to reconnect the child sender for the next iteration (note this logic also applies to successful completion).
Fixed all of the above by handling completion of the child operation as follows: