Work around Windows-only problems

The Unifex unit test suite won't build for Windows with async stack
injection enabled *unless* PR #619 (Make any_sender_of<> play nicer with
MSVC) is also merged, but that PR causes Windows + Clang + ASAN errors
in Meta-internal builds.

This diff works around the above conflict by disabling async stack
injection in Windows builds by default so we don't need PR #619. We can
change the default once we figure out a proper resolution to the ASAN
problem.

include/unifex/config.hpp

@@ -293,7 +293,21 @@
 #endif
 
 #if defined(__has_builtin)
-#define UNIFEX_HAS_BUILTIN(...) __has_builtin(__VA_ARGS__)
+#  define UNIFEX_HAS_BUILTIN(...) __has_builtin(__VA_ARGS__)
 #else
-#define UNIFEX_HAS_BUILTIN(...) 0
+#  define UNIFEX_HAS_BUILTIN(...) 0
 #endif
+
+#if !defined(UNIFEX_NO_ASYNC_STACKS)
+// default:
+//  - release builds do not have async stacks
+//  - Windows builds do not have async stacks
+//
+// adding async stacks adds non-trivial binary size at the moment, and I can't
+// figure out how to make all the relevant Windows builds succeed
+#  if defined(NDEBUG) || defined(_MSC_VER)
+#    define UNIFEX_NO_ASYNC_STACKS 1
+#  else
+#    define UNIFEX_NO_ASYNC_STACKS 0
+#  endif
+#endif  // !defined(UNIFEX_NO_ASYNC_STACKS)

include/unifex/sender_concepts.hpp

@@ -272,12 +272,23 @@ struct _fn {
     }
   };
 
+#if UNIFEX_NO_ASYNC_STACKS
+public:
+  template(typename S, typename R)  //
+      (requires sender<S> AND receiver<R>) //
+      auto
+      operator()(S&& s, R&& r) const
+          noexcept(noexcept(_impl{}(std::forward<S>(s), std::forward<R>(r))))
+          -> decltype(_impl{}(std::forward<S>(s), std::forward<R>(r))) {
+    return _impl{}(std::forward<S>(s), std::forward<R>(r));
+  }
+#else
   template <typename S, typename R>
   using op_t = _inject::op_wrapper<std::invoke_result_t<_impl, S, _inject::receiver_t<R>>, R>;
 
 public:
   template(typename S, typename R)  //
-      (requires sender<S> AND receiver<R>)
+      (requires sender<S> AND receiver<R>) //
       auto
       operator()(S&& s, R&& r) const
           noexcept(noexcept(_inject::make_op_wrapper(std::forward<S>(s), std::forward<R>(r), _impl{})))
@@ -287,6 +298,7 @@ struct _fn {
         std::forward<R>(r),
         _impl{});
   }
+#endif
 };
 
 }  // namespace _cpo

include/unifex/tracing/inject_async_stack.hpp

@@ -16,6 +16,9 @@
 #pragma once
 
 #include <unifex/config.hpp>
+
+#if !UNIFEX_NO_ASYNC_STACKS
+
 #include <unifex/receiver_concepts.hpp>
 #include <unifex/tracing/async_stack.hpp>
 #include <unifex/tracing/get_async_stack_frame.hpp>
@@ -235,3 +238,5 @@ auto make_op_wrapper(S&& s, R&& r, Fn&& fn) noexcept(
 }  // namespace unifex
 
 #include <unifex/detail/epilogue.hpp>
+
+#endif // !UNIFEX_NO_ASYNC_STACKS