try to fix weak linking on Windows

Author: ericniebler

include/stdexec/__detail/__parallel_scheduler_backend.hpp

@@ -20,7 +20,6 @@
 #include "__execution_fwd.hpp"
 
 // include these after __execution_fwd.hpp
-// #include "any_allocator.cuh"
 #include "../functional.hpp" // IWYU pragma: keep for __with_default
 #include "../stop_token.hpp" // IWYU pragma: keep for get_stop_token_t
 #include "__any_allocator.hpp"
@@ -83,28 +82,47 @@ namespace STDEXEC {
       virtual void execute(size_t, size_t) noexcept = 0;
     };
 
-    /// Interface for the parallel scheduler backend.
-    struct parallel_scheduler_backend {
-      virtual ~parallel_scheduler_backend() = 0;
-
-      /// Schedule work on parallel scheduler, calling `__r` when done and using `__s` for preallocated
-      /// memory.
-      virtual void schedule(receiver_proxy&, std::span<std::byte>) noexcept = 0;
-
-      /// Schedule bulk work of size `__n` on parallel scheduler, calling `__r` for different
-      /// subranges of [0, __n), and using `__s` for preallocated memory.
-      virtual void
-        schedule_bulk_chunked(size_t, bulk_item_receiver_proxy&, std::span<std::byte>) noexcept = 0;
-
-      /// Schedule bulk work of size `__n` on parallel scheduler, calling `__r` for each item, and
-      /// using `__s` for preallocated memory.
-      virtual void schedule_bulk_unchunked(
-        size_t,
-        bulk_item_receiver_proxy&,
-        std::span<std::byte>) noexcept = 0;
+    // This base class associates the STDEXEC::system_context_replaceability namespace
+    // with the parallel_scheduler_backend for the purposes of ADL.
+    struct __parallel_scheduler_backend_base {
+      virtual ~__parallel_scheduler_backend_base() = 0;
     };
 
-    inline parallel_scheduler_backend::~parallel_scheduler_backend() = default;
+    inline __parallel_scheduler_backend_base::~__parallel_scheduler_backend_base() = default;
+  } // namespace system_context_replaceability
+} // namespace STDEXEC
+
+// Give parallel_scheduler_backend a mangled name that does not depend on the value of the
+// STDEXEC macro. This is so that we can use weak linking to make
+// query_parallel_scheduler_backend replaceable.
+namespace __system_context_replaceability {
+  using namespace STDEXEC::system_context_replaceability;
+
+  /// Interface for the parallel scheduler backend.
+  struct parallel_scheduler_backend : __parallel_scheduler_backend_base {
+    /// Schedule work on parallel scheduler, calling `__r` when done and using `__s` for preallocated
+    /// memory.
+    virtual void schedule(receiver_proxy&, std::span<std::byte>) noexcept = 0;
+
+    /// Schedule bulk work of size `__n` on parallel scheduler, calling `__r` for different
+    /// subranges of [0, __n), and using `__s` for preallocated memory.
+    virtual void schedule_bulk_chunked(
+      std::size_t,
+      bulk_item_receiver_proxy&,
+      std::span<std::byte>) noexcept = 0;
+
+    /// Schedule bulk work of size `__n` on parallel scheduler, calling `__r` for each item, and
+    /// using `__s` for preallocated memory.
+    virtual void schedule_bulk_unchunked(
+      std::size_t,
+      bulk_item_receiver_proxy&,
+      std::span<std::byte>) noexcept = 0;
+  };
+} // namespace __system_context_replaceability
+
+namespace STDEXEC {
+  namespace system_context_replaceability {
+    using namespace ::__system_context_replaceability;
   } // namespace system_context_replaceability
 
   namespace __detail {

include/stdexec/__detail/__system_context_default_impl_entry.hpp

@@ -29,30 +29,31 @@
 
 #include "__system_context_default_impl.hpp" // IWYU pragma: keep
 
-namespace STDEXEC::system_context_replaceability {
+namespace __system_context_replaceability {
 
 #if STDEXEC_MSVC()
   /// Get the backend for the parallel scheduler.
   /// Users might replace this function.
-  extern "C" STDEXEC_SYSTEM_CONTEXT_INLINE //
-  auto __default_query_parallel_scheduler_backend() -> std::shared_ptr<parallel_scheduler_backend> {
+  STDEXEC_SYSTEM_CONTEXT_INLINE auto __default_query_parallel_scheduler_backend() //
+    -> std::shared_ptr<parallel_scheduler_backend> {
     return __system_context_default_impl::__parallel_scheduler_backend_singleton
       .__get_current_instance();
   }
 
 // If query_parallel_scheduler_backend is defined by the user, it will override the
 // default implementation. If not, the linker will resolve to the default implementation.
-#  pragma comment(                                                                                 \
-    linker,                                                                                        \
-    "/alternatename:_query_parallel_scheduler_backend=___default_query_parallel_scheduler_backend")
+#pragma comment(linker, "/alternatename:"
+  "?query_parallel_scheduler_backend@__system_context_replaceability@@YA?AV?$shared_ptr@Uparallel_scheduler_backend@__system_context_replaceability@@@std@@XZ"
+  "="
+  "?__default_query_parallel_scheduler_backend@__system_context_replaceability@@YA?AV?$shared_ptr@Uparallel_scheduler_backend@__system_context_replaceability@@@std@@XZ")
 
 #else // ^^^ MSVC ^^^ / vvv non-MSVC vvv
 
   /// Get the backend for the parallel scheduler.
   /// Users might replace this function.
   extern STDEXEC_SYSTEM_CONTEXT_INLINE STDEXEC_ATTRIBUTE(weak) //
     auto query_parallel_scheduler_backend() -> std::shared_ptr<parallel_scheduler_backend> {
-    return __system_context_default_impl::__parallel_scheduler_backend_singleton
+    return STDEXEC::__system_context_default_impl::__parallel_scheduler_backend_singleton
       .__get_current_instance();
   }
 #endif
@@ -63,8 +64,7 @@ namespace STDEXEC::system_context_replaceability {
   extern STDEXEC_SYSTEM_CONTEXT_INLINE //
   auto set_parallel_scheduler_backend(__parallel_scheduler_backend_factory_t __new_factory)
     -> __parallel_scheduler_backend_factory_t {
-    return __system_context_default_impl::__parallel_scheduler_backend_singleton
+    return STDEXEC::__system_context_default_impl::__parallel_scheduler_backend_singleton
       .__set_backend_factory(__new_factory);
   }
-
-} // namespace STDEXEC::system_context_replaceability
+} // namespace __system_context_replaceability

include/stdexec/__detail/__system_context_replaceability_api.hpp

@@ -21,17 +21,16 @@
 
 #include <memory>
 
-namespace STDEXEC::system_context_replaceability {
-  /// The type of a factory that can create `parallel_scheduler_backend` instances.
-  /// TODO(ericniebler): NOT TO SPEC.
-  using __parallel_scheduler_backend_factory_t = std::shared_ptr<parallel_scheduler_backend> (*)();
-
+namespace __system_context_replaceability {
   /// Get the backend for the parallel scheduler.
   /// Users might replace this function.
-  STDEXEC_PP_WHEN(STDEXEC_MSVC(), extern "C")
   STDEXEC_ATTRIBUTE(weak) auto query_parallel_scheduler_backend()
     -> std::shared_ptr<parallel_scheduler_backend>;
 
+  /// The type of a factory that can create `parallel_scheduler_backend` instances.
+  /// NOT TO SPEC
+  using __parallel_scheduler_backend_factory_t = std::shared_ptr<parallel_scheduler_backend> (*)();
+
   /// Set a factory for the parallel scheduler backend.
   /// Can be used to replace the parallel scheduler at runtime.
   /// NOT TO SPEC
@@ -41,4 +40,4 @@ namespace STDEXEC::system_context_replaceability {
     "instead.")]]
   auto set_parallel_scheduler_backend(__parallel_scheduler_backend_factory_t __new_factory)
     -> __parallel_scheduler_backend_factory_t;
-} // namespace STDEXEC::system_context_replaceability
+} // namespace __system_context_replaceability