This allows e.g. the following programming pattern:
```
RecursiveCoroutine<std::optional<SomeType>> f();
int g() {
return *f();
}
```
Without operator* defined for `RecursiveCoroutine` this would fail,
requiring an explicit cast such as:
```
int g() {
return *static_cast<std::optional<SomeType>>(f());
}
```
Fix compilation error: the expression 'co_await
__promise.final_suspend()' is required to be non-throwing
This compilation error started being triggered when switching to
clang-12.
Before this commit, the execution logic of RecursiveCoroutine used an
underlying std::stack allocated on the heap to hold all the
coroutine_handles. It also manually managed passing return values from
callees to callers.
This commit drops this unnecessary auxiliary stack.
When a RecursiveCoroutine co_awaits another one, the handle of the
awaiter is injected into the awaitee, so that when the awaitee is done
it can directly execute the remaining part of the awaiter as a
continuation.
Before this commit, recursive coroutines did not work properly if they
had out arguments with reference type.
The reason is that `rc_run` was inferring the type of its arguments from
the arguments themselves, not from the prototype of the recursive
coroutine.
Hence, code snippets like the following did not work properly, because
`rc_run` was taking x by value, not by reference.
```
RecursiveCoroutine<void> accumulate_on_i(int &i) {
// ...
}
int f() {
int x = 0;
rc_run(accumulate_on_i, x);
return x;
}
```
This commit fixes the problem. Now the arguments of `rc_run` are
properly forwarded to the recursive coroutine.
RecursiveCoroutines are a facility intended to be used as-drop in
replacement of recursive functions.
They provide the following features.
- They can be written almost as regular recursive functions,
with 4 caveats.
1. A recursive coroutine that returns a type `T`, needs to be declared
to return a `RecursiveCoroutine<T>`.
2. Inside the body of a recursive coroutine, when recursively calling
another recursive coroutine, the recursive call needs to be
prepended by the new keyword `rc_recur`.
3. Inside the body of a recursive coroutine, the `return` statement
needs to be substituted with `rc_return`.
4. When launching a recursive coroutine `A` from a function that is
not a recursive coroutine, `A` needs to be called with the provided
dedicated template wrapper `rc_run`.
The syntax is the following `rc_run(A, arg0, arg1, ...)`.
This is necessary to enable swapping off recursive coroutine and
fall back to regular recursion for debug.
See below for how to do it.
- Unlike regular recursive functions, they don't use the system stack
for recursion. They use a custom heap-allocated stack to manage
recursion. This makes them more robust for implementing recursive
functions that manipulate user-defined input, because they are much
less likely to trigger stack overflow.
- They can be turned off compiling with
`-DDISABLE_RECURSIVE_COROUTINES`, falling back to regular recursion,
for debug purposes.
- They support both direct and indirect recursion, i.e. a recursive
coroutine A can recursively call itself, or it can recursively call
another recursive coroutine B, which in turns recursively calls A.
Alessandro Di Federico