[ChangeLog] Add an entry describing new behavior when passing type name as an argument

Author: xedin

CHANGELOG.md

@@ -6,6 +6,24 @@ _**Note:** This is in reverse chronological order, so newer entries are added to
 Swift 5.5
 ---------
 
+* Type names are no longer allowed as an argument to a subscript parameter that expects a metatype type
+
+```swift
+struct MyValue {
+}
+
+struct MyStruct {
+  subscript(a: MyValue.Type) -> Int { get { ... } }
+}
+
+func test(obj: MyStruct) {
+  let _ = obj[MyValue]
+}
+```
+
+Accepting subscripts with `MyValue` as an argument was an oversight because `MyValue` requires explicit `.self`
+to reference its metatype, so correct syntax would be to use `obj[MyValue.self]`.
+
 * [SE-0310][]:
 
   Read-only computed properties and subscripts can now define their `get` accessor to be `async` and/or `throws`, by writing one or both of those keywords between the `get` and `{`.  Thus, these members can now make asynchronous calls or throw errors in the process of producing a value:
@@ -49,7 +67,6 @@ Swift 5.5
   }
   ```
 
-
 * [SE-0306][]:
 
   Swift 5.5 includes support for actors, a new kind of type that isolates its instance data to protect it from concurrent access. Accesses to an actor's instance declarations from outside the must be asynchronous: