fix confusing protocol diagnostic

- update the diagnostic error message
- add educational notes

Author: OnyekachiSamuel

include/swift/AST/DiagnosticsSema.def

@@ -1803,8 +1803,8 @@ ERROR(use_of_equal_instead_of_equality,none,
       "use of '=' in a boolean context, did you mean '=='?", ())
 
 ERROR(type_cannot_conform, none,
-      "%select{|value of protocol }0type %1 cannot conform to %2; "
-      "only struct/enum/class types can conform to protocols",
+      "%select{|A value of protocol }0type %1 does not conform to the protocol itself (exception: 'Error'); "
+      "only concrete types such as structs, enums and classes can conform to protocols",
       (bool, Type, Type))
 NOTE(required_by_opaque_return,none,
     "required by opaque return type of %0 %1", (DescriptiveDeclKind, DeclName))

include/swift/AST/EducationalNotes.def

@@ -65,5 +65,6 @@ EDUCATIONAL_NOTES(append_interpolation_static,
                   "string-interpolation-conformance.md")
 EDUCATIONAL_NOTES(append_interpolation_void_or_discardable,
                   "string-interpolation-conformance.md")
+EDUCATIONAL_NOTES(type_cannot_conform, "protocol-type-non-conformance.md")                
 
 #undef EDUCATIONAL_NOTES

userdocs/diagnostics/protocol-type-non-conformance.md

@@ -0,0 +1,41 @@
+# Protocol type not conforming to itself
+Swift disallows us from using a protocol as a type that conforms to itself as illustrated in the examples below
+
+```swift
+protocol SomeProtocol {
+    init()
+}
+
+struct SomeStruct: SomeProtocol {}
+struct AnotherStruct: SomeProtocol {}
+
+var arr: [SomeProtocol] = [SomeStruct(), AnotherStruct()]
+arr.appendNewItem()
+
+extension Array where Element: SomeProtocol {
+    mutating func appendNewItem() {
+        append(Element())
+    }
+}
+```
+
+The code snippet above would not compile because we are using `SomeProtocol` as a type that conforms to itself. There is no concrete implementation for the protocol.
+
+Consider also the case of using protocol as a type in a generic type - 
+
+```swift
+protocol AnotherProtocol {
+  static func foo()
+}
+
+struct GenericStruct<T: AnotherProtocol> {
+    func faz() {
+        T.foo()
+    }
+}
+
+GenericStruct<AnotherProtocol>().faz()
+```
+Constructing the instance of the struct `GenericStruct` with type `AnotherProtocol` will not compile because there is no concrete implementation for the static requirement of the protocol. 
+There is no implementation for for() used above.
+