Specify defensive copy for other readonly variables

Addresses dotnet#894 (comment)

Author: BillWagner

standard/expressions.md

@@ -1211,7 +1211,11 @@ The run-time processing of a function member invocation consists of the followin
   - `M` is invoked.
 - Otherwise, if the type of `E` is a value-type `V`, and `M` is declared or overridden in `V`:
   - `E` is evaluated. If this evaluation causes an exception, then no further steps are executed. For an instance constructor, this evaluation consists of allocating storage (typically from an execution stack) for the new object. In this case `E` is classified as a variable.
-  - If `E` is not classified as a variable, or if `E` is an input parameter (§15.6.2.3) and `V` is not a readonly struct type (§16.2.2), then a temporary local variable of `E`’s type is created and the value of `E` is assigned to that variable. `E` is then reclassified as a reference to that temporary local variable. The temporary variable is accessible as `this` within `M`, but not in any other way. Thus, only when `E` is a true variable is it possible for the caller to observe the changes that `M` makes to `this`.
+  - If `E` is not classified as a variable, or if `V` is not a readonly struct type (§16.2.2), and `E` is one of:
+    - an input parameter (§15.6.2.3), or
+    - a `readonly` field (§15.5.3), or
+    - a `readonly` reference variable or return (§9.7), 
+    then a temporary local variable of `E`’s type is created and the value of `E` is assigned to that variable. `E` is then reclassified as a reference to that temporary local variable. The temporary variable is accessible as `this` within `M`, but not in any other way. Thus, only when `E` is a true variable is it possible for the caller to observe the changes that `M` makes to `this`.
   - The argument list is evaluated as described in [§12.6.2](expressions.md#1262-argument-lists).
   - `M` is invoked. The variable referenced by `E` becomes the variable referenced by `this`.
 - Otherwise: