Give struct __rcvr user-defined ctors in __basic_sender (NVIDIA#2102)

This looks like it fixes a miscompilation leading to UB in combinations
of `read_env` and `write_env` in Apple Clang.

Author: ispeters

include/stdexec/__detail/__basic_sender.hpp

@@ -236,6 +236,19 @@ namespace STDEXEC
     using receiver_concept = receiver_tag;
     using __index_t        = __msize_t<_Idx>;
 
+#if STDEXEC_APPLE_CLANG()
+    // These constructors are a work-around for bad codegen with apple-clang
+    STDEXEC_ATTRIBUTE(always_inline)
+    constexpr explicit __rcvr(_State& __state) noexcept
+      : __state_(__state)
+    {}
+
+    STDEXEC_ATTRIBUTE(always_inline)
+    constexpr __rcvr(__rcvr const & __other) noexcept
+      : __state_(__other.__state_)
+    {}
+#endif  // STDEXEC_APPLE_CLANG()
+
     template <class... _Args>
     STDEXEC_ATTRIBUTE(always_inline)
     constexpr void set_value(_Args&&... __args) noexcept