Add workaround for MSVC vs. CapPtr constructor

src/ds/ptrwrap.h

@@ -112,8 +112,22 @@ namespace snmalloc
 
     /**
      * all other constructions must be explicit
+     *
+     * Unfortunately, MSVC gets confused if an Allocator is instantiated in a
+     * way that never needs initialization (as our sandbox test does, for
+     * example) and, in that case, declares this constructor unreachable,
+     * presumably after some heroic feat of inlining that has also lost any
+     * semblance of context.  See the blocks tagged "CapPtr-vs-MSVC" for where
+     * this has been observed.
      */
+#ifdef _MSC_VER
+#  pragma warning(push)
+#  pragma warning(disable : 4702)
+#endif
     explicit CapPtr(T* p) : unsafe_capptr(p) {}
+#ifdef _MSC_VER
+#  pragma warning(pop)
+#endif
 
     /**
      * Allow static_cast<>-s that preserve bounds but vary the target type.

src/test/func/sandbox/sandbox.cc

@@ -19,6 +19,8 @@ namespace
   /**
    * Helper for Alloc that is never used as a thread-local allocator and so is
    * always initialised.
+   *
+   * CapPtr-vs-MSVC triggering; xref CapPtr's constructor
    */
   bool never_init(void*)
   {
@@ -26,6 +28,8 @@ namespace
   }
   /**
    * Helper for Alloc that never needs lazy initialisation.
+   *
+   * CapPtr-vs-MSVC triggering; xref CapPtr's constructor
    */
   void* no_op_init(function_ref<void*(void*)>)
   {