LWG3460 Unimplementable noop_coroutine_handle guarantees

For clarification, "returns the same value" was augmented to "returns
the same non-null value", on suggestion of and with review by LWG
members.

Author: burblebee

source/support.tex

@@ -5137,18 +5137,35 @@
   };
 
   template<class Promise>
-  struct coroutine_handle : coroutine_handle<>
+  struct coroutine_handle
   {
     // \ref{coroutine.handle.con}, construct/reset
-    using coroutine_handle<>::coroutine_handle;
+    constexpr coroutine_handle() noexcept;
+    constexpr coroutine_handle(nullptr_t) noexcept;
     static coroutine_handle from_promise(Promise&);
     coroutine_handle& operator=(nullptr_t) noexcept;
 
     // \ref{coroutine.handle.export.import}, export/import
+    constexpr void* address() const noexcept;
     static constexpr coroutine_handle from_address(void* addr);
 
+    // \ref{coroutine.handle.conv}, conversion
+    constexpr operator coroutine_handle<>() const noexcept;
+
+    // \ref{coroutine.handle.observers}, observers
+    constexpr explicit operator bool() const noexcept;
+    bool done() const;
+
+    // \ref{coroutine.handle.resumption}, resumption
+    void operator()() const;
+    void resume() const;
+    void destroy() const;
+
     // \ref{coroutine.handle.promise}, promise access
     Promise& promise() const;
+
+  private:
+    void* ptr;  // \expos
   };
 }
 \end{codeblock}
@@ -5157,8 +5174,12 @@
 An object of type
 \tcode{coroutine_handle<T>} is called a \term{coroutine handle}
 and can be used to refer to a suspended or executing coroutine.
-A default-constructed \tcode{coroutine_handle} object does not refer to any
+A \tcode{coroutine_handle} object whose
+member \tcode{address()} returns a null pointer value
+does not refer to any
 coroutine.
+Two \tcode{coroutine_handle} objects refer to the same coroutine
+if and only if their member \tcode{address()} returns the same non-null value.
 
 \pnum
 If a program declares an explicit or partial specialization of
@@ -5212,6 +5233,19 @@
 \tcode{*this}.
 \end{itemdescr}
 
+\rSec3[coroutine.handle.conv]{Conversion}
+
+\indexlibrarymember{operator coroutine_handle<>}{coroutine_handle}%
+\begin{itemdecl}
+constexpr operator coroutine_handle<>() const noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to: \tcode{return coroutine_handle<>::from_address(address());}
+\end{itemdescr}
+
 \rSec3[coroutine.handle.export.import]{Export/import}
 
 \indexlibrarymember{address}{coroutine_handle}%
@@ -5228,13 +5262,29 @@
 \indexlibrarymember{from_address}{coroutine_handle}%
 \begin{itemdecl}
 static constexpr coroutine_handle<> coroutine_handle<>::from_address(void* addr);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\expects
+\tcode{addr} was obtained via a prior call to \tcode{address}
+on an object whose type is a specialization of \tcode{coroutine_handle}.
+
+\pnum
+\ensures
+\tcode{from_address(address()) == *this}.
+\end{itemdescr}
+
+\indexlibrarymember{from_address}{coroutine_handle}%
+\begin{itemdecl}
 static constexpr coroutine_handle<Promise> coroutine_handle<Promise>::from_address(void* addr);
 \end{itemdecl}
 
 \begin{itemdescr}
 \pnum
 \expects
-\tcode{addr} was obtained via a prior call to \tcode{address}.
+\tcode{addr} was obtained via a prior call to \tcode{address}
+on an object of type \cv \tcode{coroutine_handle<Promise>}.
 
 \pnum
 \ensures
@@ -5397,8 +5447,11 @@
 \begin{codeblock}
 namespace std {
   template<>
-  struct coroutine_handle<noop_coroutine_promise> : coroutine_handle<>
+  struct coroutine_handle<noop_coroutine_promise>
   {
+    // \ref{coroutine.handle.noop.conv}, conversion
+    constexpr operator coroutine_handle<>() const noexcept;
+
     // \ref{coroutine.handle.noop.observers}, observers
     constexpr explicit operator bool() const noexcept;
     constexpr bool done() const noexcept;
@@ -5415,10 +5468,24 @@
     constexpr void* address() const noexcept;
   private:
     coroutine_handle(@\unspec@);
+    void* ptr;  // \expos
   };
 }
 \end{codeblock}
 
+\rSec4[coroutine.handle.noop.conv]{Conversion}
+
+\indexlibrarymember{operator coroutine_handle<>}{coroutine_handle<noop_coroutine_promise>}%
+\begin{itemdecl}
+constexpr operator coroutine_handle<>() const noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to: \tcode{return coroutine_handle<>::from_address(address());}
+\end{itemdescr}
+
 \rSec4[coroutine.handle.noop.observers]{Observers}
 
 \indexlibrarymember{operator bool}{coroutine_handle<noop_coroutine_promise>}%