allow for stdexec::type_info to be expanded in the future

.clangd

@@ -40,6 +40,9 @@ CompileFlags:
     - "-Wno-unknown-cuda-version"
     - "--cuda-host-only"
     - "--cuda-path=/usr/local/cuda-12"
+  Remove:
+    - "-x"
+    - "c++-header"
 
 ---
 
@@ -54,7 +57,6 @@ CompileFlags:
     - "-ftemplate-backtrace-limit=0"
     - "-std=c++20"
     - "-DSTDEXEC_CLANGD_INVOKED"
-    - "-stdlib=libc++"
     - "-Wno-unused-local-typedef"
   Remove:
     - "-stdpar*"

include/stdexec/__detail/__config.hpp

@@ -602,6 +602,19 @@ namespace STDEXEC
 #  define STDEXEC_NO_STDCPP_EXPLICIT_THIS_PARAMETER() 1
 #endif
 
+#if defined(__cpp_rtti) && __cpp_rtti >= 1997'11L
+#  define STDEXEC_NO_STDCPP_RTTI() 0
+#else
+#  define STDEXEC_NO_STDCPP_RTTI() 1
+#endif
+
+// MSVC always has typeid support, even when RTTI is disabled
+#if STDEXEC_NO_STDCPP_RTTI() && !STDEXEC_MSVC()
+#  define STDEXEC_NO_STDCPP_TYPEID() 1
+#else
+#  define STDEXEC_NO_STDCPP_TYPEID() 0
+#endif
+
 // Perhaps the stdlib lacks support for concepts
 #if __has_include(<concepts>) && __cpp_lib_concepts >= 2020'02L
 #  define STDEXEC_NO_STDCPP_CONCEPTS_HEADER() 0

include/stdexec/__detail/__parallel_scheduler_backend.hpp

@@ -25,6 +25,7 @@
 #include "__any_allocator.hpp"
 #include "__optional.hpp"
 #include "__queries.hpp"
+#include "__schedulers.hpp"
 #include "__typeinfo.hpp"
 
 #include <exception>

include/stdexec/__detail/__typeinfo.hpp

@@ -23,6 +23,10 @@
 
 #include <compare>
 #include <string_view>
+#include <typeinfo>
+
+STDEXEC_PRAGMA_PUSH()
+STDEXEC_PRAGMA_IGNORE_GNU("-Wunused-private-field")
 
 //////////////////////////////////////////////////////////////////////////////////////////
 // __type_info, __mtypeid, and __mtypeof
@@ -36,12 +40,14 @@ namespace STDEXEC
     constexpr __type_info(__type_info &&)            = delete;
     constexpr __type_info &operator=(__type_info &&) = delete;
 
-    constexpr explicit __type_info(std::string_view __name) noexcept
+    constexpr explicit __type_info(std::string_view      __name,
+                                   std::type_info const *__type = nullptr) noexcept
       : __name_(__name)
+      , __type_(__type)
     {}
 
     [[nodiscard]]
-    constexpr std::string_view name() const noexcept
+    constexpr auto name() const noexcept -> std::string_view
     {
       return __name_;
     }
@@ -52,17 +58,38 @@ namespace STDEXEC
       return this == &__other || __name_ == __other.__name_;
     }
 
-    constexpr auto
-    operator<=>(__type_info const &) const noexcept -> std::strong_ordering = default;
+    constexpr auto operator<=>(__type_info const &__other) const noexcept -> std::strong_ordering
+    {
+      return __name_ <=> __other.__name_;
+    }
+
+#if !STDEXEC_NO_STDCPP_TYPEID()
+    [[nodiscard]]
+    constexpr auto type() const noexcept -> std::type_info const &
+    {
+      return *__type_;
+    }
+
+    [[nodiscard]]
+    constexpr operator std::type_info const &() const noexcept
+    {
+      return *__type_;
+    }
+#endif
 
    private:
-    std::string_view __name_;
+    std::string_view const      __name_;
+    std::type_info const *const __type_     = nullptr;  // used only if !STDEXEC_NO_STDCPP_TYPEID()
+    void const *const           __reserved_ = nullptr;  // reserved for future use
   };
 
   namespace __detail
   {
     template <class _Ty>
-    inline constexpr __type_info __mtypeid_v{__mnameof<_Ty>};
+    inline constexpr __type_info __mtypeid_v{
+      __mnameof<_Ty>                                                                 //
+        STDEXEC_PP_WHEN(STDEXEC_PP_NOT(STDEXEC_NO_STDCPP_TYPEID()), , &typeid(_Ty))  //
+    };
 
     template <class _Ty>
     inline constexpr __type_info const &__mtypeid_v<_Ty const> = __mtypeid_v<_Ty>;
@@ -94,6 +121,20 @@ namespace STDEXEC
       return *__info_ <=> *other.__info_;
     }
 
+#if !STDEXEC_NO_STDCPP_TYPEID()
+    [[nodiscard]]
+    constexpr auto type() const noexcept -> std::type_info const &
+    {
+      return (*__info_).type();
+    }
+
+    [[nodiscard]]
+    constexpr operator std::type_info const &() const noexcept
+    {
+      return (*__info_).type();
+    }
+#endif
+
     __type_info const *__info_;
   };
 
@@ -139,3 +180,5 @@ namespace STDEXEC
   // Sanity check:
   static_assert(STDEXEC_IS_SAME(void, __mtypeof<__mtypeid<void>>));
 }  // namespace STDEXEC
+
+STDEXEC_PRAGMA_POP()

include/stdexec/__detail/__utility.hpp

@@ -312,7 +312,7 @@ namespace STDEXEC
     static_assert(std::derived_from<__value_type, _CvInterface>,
                   "__polymorphic_downcast requires From to be a base class of To");
 
-#if defined(__cpp_rtti) && __cpp_rtti >= 1997'11L
+#if !STDEXEC_NO_STDCPP_RTTI()
     STDEXEC_IF_NOT_CONSTEVAL
     {
       STDEXEC_ASSERT(dynamic_cast<__value_type*>(__from_ptr) != nullptr);