Maybe fix MSVC + CUDA builds

The MSVC build failure on the previous commit looks like a misplaced
`[[no_unique_address]]`; the CUDA build failures are mysterious to me,
but they appear to be downstream of changing `__any`'s `__box` type to
use in-place new into an uninitialized member of an anonymous union,
which I did to address a GCC build failure. This diff makes the
anonymous union hack GCC-specific, to hopefull make all the compilers
happy at the expense of preprocessor complexity.

Author: ispeters

include/stdexec/__detail/__any.hpp

@@ -465,11 +465,25 @@ namespace STDEXEC::__any
       : __val_(static_cast<_Args &&>(__args)...)
     {}
 
+#if !STDEXEC_GCC()
     template <class _Fn, class... _Args>
     constexpr explicit __box(__in_place_from_t, _Fn &&__fn, _Args &&...__args)
       noexcept(__nothrow_callable<_Fn, _Args...>)
+      : __val_(static_cast<_Fn &&>(__fn)(static_cast<_Args &&>(__args)...))
     {
       static_assert(__same_as<__call_result_t<_Fn, _Args...>, _Value>);
+    }
+#else
+    template <class _Fn, class... _Args>
+    constexpr explicit __box(__in_place_from_t, _Fn &&__fn, _Args &&...__args)
+      noexcept(__nothrow_callable<_Fn, _Args...>)
+    {
+      // for unknown reasons, GCC (sometimes) doesn't like initializing __val_ with the
+      // result of this function call in the member initialization clause when _Value
+      // is immovable even though the conditions should be right to trigger C++17's
+      // mandatory copy elision, but this in-place new into an uninitialized anonymous
+      // union member works just fine
+      static_assert(__same_as<__call_result_t<_Fn, _Args...>, _Value>);
       new ((void *) std::addressof(__val_))
         _Value(static_cast<_Fn &&>(__fn)(static_cast<_Args &&>(__args)...));
     }
@@ -502,6 +516,7 @@ namespace STDEXEC::__any
       __val_ = __rhs.__val_;
       return *this;
     }
+#endif
 
     template <class _Self>
     [[nodiscard]]
@@ -511,11 +526,16 @@ namespace STDEXEC::__any
     }
 
    private:
+#if !STDEXEC_GCC()
     STDEXEC_ATTRIBUTE(no_unique_address)
+    _Value __val_;
+#else
     union
     {
+      STDEXEC_ATTRIBUTE(no_unique_address)
       _Value __val_;
     };
+#endif
   };
 
   template <class _Interface, __box_kind _BoxKind>