Merged F.24 into F.15 (issue #1)

Author: hsutter

CppCoreGuidelines.md

@@ -1785,7 +1785,6 @@ Argument passing rules:
 * [F.17: Use a `not_null<T>` to indicate "null" is not a valid value](#Rf-nullptr)
 * [F.18: Use a `span<T>` or a `span_p<T>` to designate a half-open sequence](#Rf-range)
 * [F.19: Use a `zstring` or a `not_null<zstring>` to designate a C-style string](#Rf-string)
-* [F.24: Use a `TP&&` parameter when forwarding (only)](#Rf-pass-ref-ref)
 * [F.25: Use a `T&&` parameter together with `move` for rare optimization opportunities](#Rf-pass-ref-move)
 * [F.26: Use a `unique_ptr<T>` to transfer ownership where a pointer is needed](#Rf-unique_ptr)
 * [F.27: Use a `shared_ptr<T>` to share ownership](#Rf-shared_ptr)
@@ -2181,9 +2180,9 @@ Pure functions are easier to reason about, sometimes easier to optimize (and eve
 
 Not possible.
 
-## <a name="SS-call"></a> F.call: Argument passing
+## <a name="SS-call"></a> F.call: Parameter passing
 
-There are a variety of ways to pass arguments to a function and to return values.
+There are a variety of ways to pass parameters to a function and to return values.
 
 ### <a name="Rf-conventional"></a> Rule F.15: Prefer simple and conventional ways of passing information
 
@@ -2314,12 +2313,24 @@ A reference may be assumed to refer to a valid object (language rule).
 There is no (legitimate) "null reference."
 If you need the notion of an optional value, use a pointer, `std::optional`, or a special value used to denote "no value."
 
+
+**For an "forwarded" value:** If the object is to be passed onward to other code and not directly used by this function, we want to make this function agnostic to the argument `const`-ness and rvalue-ness. In that case, and only that case, make the parameter `TP&&` where `TP` is a template type parameter -- it both *ignores* and *preserves* `const`-ness and rvalue-ness. Therefore any code that uses a `T&&` is implicitly declaring that it itself doesn't care about the variable's `const`'-ness and rvalue-ness (because it is ignored), but that intends to pass the value onward to other code that does care about `const`-ness and rvalue-ness (because it is preserved). When used as a parameter `TP&&` is safe because any temporary objects passed from the caller will live for the duration of the function call. A parameter of type `TP&&` should essentially always be passed onward via `std::forward` in the body of the function.
+
+##### Example
+
+    template <class F, class... Args>
+    inline auto invoke(F&& f, Args&&... args) {
+        return forward<F>(f)(forward<Args>(args)...);
+    }
+ 
+
 ##### Enforcement
 
 * (Simple) ((Foundation)) Warn when a parameter being passed by value has a size greater than `4 * sizeof(int)`.
   Suggest using a `const` reference instead.
 * (Simple) ((Foundation)) Warn when a `const` parameter being passed by reference has a size less than `3 * sizeof(int)`. Suggest passing by value instead.
 * (Moderate) ((Foundation)) Warn about functions with non-`const` reference arguments that do *not* write to them.
+* Flag a function that takes a `TP&&` parameter (where `TP` is a template type parameter name) and uses it without `std::forward`.
 
 
 **See also**: [implicit arguments](#Ri-explicit).
@@ -2458,23 +2469,6 @@ When I call `length(s)` should I test for `s == nullptr` first? Should the imple
 **See also**: [Support library](#S-gsl).
 
 
-### <a name="Rf-pass-ref-ref"></a> F.24: Use a `TP&&` parameter when forwarding (only)
-
-##### Reason
-
-When `TP` is a template type parameter, `TP&&` is a forwarding reference -- it both *ignores* and *preserves* `const`'ness and rvalue-ness. Therefore any code that uses a `T&&` is implicitly declaring that it itself doesn't care about the variable's `const`'-ness and rvalue-ness (because it is ignored), but that intends to pass the value onward to other code that does care about `const`'-ness and rvalue-ness (because it is preserved). When used as a parameter `TP&&` is safe because any temporary objects passed from the caller will live for the duration of the function call. A parameter of type `TP&&` should essentially always be passed onward via `std::forward` in the body of the function.
-
-##### Example
-
-    template <class F, class... Args>
-    inline auto invoke(F&& f, Args&&... args) {
-        return forward<F>(f)(forward<Args>(args)...);
-    }
-
-##### Enforcement
-
-Flag a function that takes a `TP&&` parameter (where `TP` is a template type parameter name) and uses it without `std::forward`.
-
 ### <a name="Rf-pass-ref-move"></a> F.25: Use a `T&&` parameter together with `move` for rare optimization opportunities
 
 ##### Reason