F.20 extract unrelated parts into new rule, F.49

closes #1862

CppCoreGuidelines.md

@@ -1,7 +1,6 @@
 # <a name="main"></a>C++ Core Guidelines
 
-September 23, 2022
-
+October 6, 2022
 
 Editors:
 
@@ -2378,6 +2377,7 @@ Parameter passing semantic rules:
 * [F.46: `int` is the return type for `main()`](#Rf-main)
 * [F.47: Return `T&` from assignment operators](#Rf-assignment-op)
 * [F.48: Don't `return std::move(local)`](#Rf-return-move-local)
+* [F.49: Don't return `const T`](#Rf-return-const)
 
 Other function rules:
 
@@ -3157,25 +3157,6 @@ If you have multiple values to return, [use a tuple](#Rf-out-multi) or similar m
 
 A `struct` of many (individually cheap-to-move) elements might be in aggregate expensive to move.
 
-##### Note
-
-It is not recommended to return a `const` value.
-Such older advice is now obsolete; it does not add value, and it interferes with move semantics.
-
-    const vector<int> fct();    // bad: that "const" is more trouble than it is worth
-
-    void g(vector<int>& vx)
-    {
-        // ...
-        fct() = vx;   // prevented by the "const"
-        // ...
-        vx = fct(); // expensive copy: move semantics suppressed by the "const"
-        // ...
-    }
-
-The argument for adding `const` to a return value is that it prevents (very rare) accidental access to a temporary.
-The argument against is that it prevents (very frequent) use of move semantics.
-
 ##### Exceptions
 
 * For non-concrete types, such as types in an inheritance hierarchy, return the object by `unique_ptr` or `shared_ptr`.
@@ -3218,7 +3199,6 @@ The return value optimization doesn't handle the assignment case, but the move a
 ##### Enforcement
 
 * Flag reference to non-`const` parameters that are not read before being written to and are a type that could be cheaply returned; they should be "out" return values.
-* Flag returning a `const` value. To fix: Remove `const` to return a non-`const` value instead.
 
 ### <a name="Rf-out-multi"></a>F.21: To return multiple "out" values, prefer returning a struct or tuple
 
@@ -3910,7 +3890,6 @@ This was primarily to avoid code of the form `(a = b) = c` -- such code is not c
 This should be enforced by tooling by checking the return type (and return
 value) of any assignment operator.
 
-
 ### <a name="Rf-return-move-local"></a>F.48: Don't `return std::move(local)`
 
 ##### Reason
@@ -3937,6 +3916,35 @@ With guaranteed copy elision, it is now almost always a pessimization to express
 
 This should be enforced by tooling by checking the return expression .
 
+### <a name="Rf-return-const"></a>F.49: Don't return `const T`
+
+##### Reason
+
+It is not recommended to return a `const` value.
+Such older advice is now obsolete; it does not add value, and it interferes with move semantics.
+
+##### Example
+
+    const vector<int> fct();    // bad: that "const" is more trouble than it is worth
+
+    void g(vector<int>& vx)
+    {
+        // ...
+        fct() = vx;   // prevented by the "const"
+        // ...
+        vx = fct(); // expensive copy: move semantics suppressed by the "const"
+        // ...
+    }
+
+The argument for adding `const` to a return value is that it prevents (very rare) accidental access to a temporary.
+The argument against is that it prevents (very frequent) use of move semantics.
+
+**See also**: [F.20, the general item about "out" output values](#Rf-out)
+
+##### Enforcement
+
+* Flag returning a `const` value. To fix: Remove `const` to return a non-`const` value instead.
+
 
 ### <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)