Add F.47 - Assignment operators return T&

As discussed in
https://github.com/isocpp/CppCoreGuidelines/issues/422

CppCoreGuidelines.md

@@ -2873,6 +2873,38 @@ Declaring `main` (the one global `main` of a program) `void` limits portability.
 * The compiler should do it
 * If the compiler doesn't do it, let tools flag it
 
+### <a name="Rf-assignment-op"></a>F.46: Return `T&` from assignment operators.
+
+##### Reason
+
+The convention for operator overloads (especially on value types) is for
+`operator=(const T&)` to perform the assignment and then return (non-const)
+`*this`.  This ensures consistency with standard library types and follows the
+principle of "do as the ints do."
+
+##### Note
+
+Historically there was some guidance to make the assignment operator return
+`const T&`.  This was primarily to avoid code of the form `(a=b)=c` - such code
+is not common enough to warrant violating consistency with standard types.
+
+##### Example
+
+        class Foo
+        {
+         public:
+            ...
+            Foo& operator=(const Foo& rhs) {
+              // Copy members.
+              ...
+              return *this;
+            }
+        };
+
+##### Enforcement
+
+This should be enforced by tooling by checking the return type (and return
+value) of any assignment operator.
 
 ### <a name="Rf-capture-vs-overload"></a> F.50: Use a lambda when a function won't do (to capture local variables, or to write a local function)