Make Coroutine.h compatible with libstdc++

Summary: Add support for both <coroutine> and <experimental/coroutine> headers. Also add support for __cpp_impl_coroutine (which is required by the standard).

Reviewed By: yfeldblum

Differential Revision: D26745085

fbshipit-source-id: 0bf932e2e30d5b105b1559d817563946fcd3b573

folly/Portability.h

@@ -562,7 +562,8 @@ constexpr auto kCpplibVer = 0;
 // <experimental/coroutine> which will conflict with anyone who wants to load
 // the LLVM implementation of coroutines on Windows.
 #define FOLLY_HAS_COROUTINES 0
-#elif __cpp_coroutines >= 201703L && __has_include(<experimental/coroutine>)
+#elif (__cpp_coroutines >= 201703L || __cpp_impl_coroutine >= 201902L) && \
+    (__has_include(<coroutine>) || __has_include(<experimental/coroutine>))
 #define FOLLY_HAS_COROUTINES 1
 // This is mainly to workaround bugs triggered by LTO, when stack allocated
 // variables in await_suspend end up on a coroutine frame.

folly/experimental/coro/Coroutine.h

@@ -27,7 +27,11 @@
 
 #if FOLLY_HAS_COROUTINES
 
+#if __has_include(<coroutine>)
+#include <coroutine>
+#else
 #include <experimental/coroutine>
+#endif
 
 #endif // FOLLY_HAS_COROUTINES
 
@@ -43,13 +47,19 @@
 
 namespace folly::coro {
 
-using std::experimental::coroutine_handle;
-using std::experimental::coroutine_traits;
-using std::experimental::noop_coroutine;
-using std::experimental::noop_coroutine_handle;
-using std::experimental::noop_coroutine_promise;
-using std::experimental::suspend_always;
-using std::experimental::suspend_never;
+#if __has_include(<coroutine>)
+namespace impl = std;
+#else
+namespace impl = std::experimental;
+#endif
+
+using impl::coroutine_handle;
+using impl::coroutine_traits;
+using impl::noop_coroutine;
+using impl::noop_coroutine_handle;
+using impl::noop_coroutine_promise;
+using impl::suspend_always;
+using impl::suspend_never;
 
 //  ready_awaitable
 //