declaration.dd: clarify void initializations, @safe, and UB

Author: WalterBright

spec/declaration.dd

@@ -608,14 +608,31 @@ $(GNAME VoidInitializer):
         used before it is set, undefined program behavior will result.
         )
 
-        $(UNDEFINED_BEHAVIOR If a void initialized variable's value is
-        used before it is set, the behavior is undefined.
+        $(IMPLEMENTATION_DEFINED If a void initialized variable's value is
+        used before it is set, the value is implementation defined.
 
         ---
+        import std.stdio;
         void foo()
         {
             int x = void;
-            writeln(x);  // will print garbage
+            writeln(x);  // will likely print garbage
+        }
+        ---
+        )
+
+        $(UNDEFINED_BEHAVIOR If a void initialized variable is a reference type
+        and its value is dereferenced before it is set, the behavior is undefined.
+        Such initializations are not allowed in `@safe` code.
+
+        ---
+        void foo()
+        {
+            int* p = void;
+            *p = 3;      // undefined behavior
+            int x;
+            p = &x;
+            *p = 3;      // ok
         }
         ---
         )