[SR-12248] Add tests

Author: HassanElDesouky

test/decl/protocol/special/coding/struct_codable_simple.swift

@@ -29,3 +29,21 @@ let _ = SimpleStruct.encode(to:)
 // The synthesized CodingKeys type should not be accessible from outside the
 // struct.
 let _ = SimpleStruct.CodingKeys.self // expected-error {{'CodingKeys' is inaccessible due to 'private' protection level}}
+
+// rdar://problem/59655704 
+struct SR_12248_1: Codable { // expected-error {{type 'SR_12248_1' does not conform to protocol 'Encodable'}}
+  var x: Int // expected-note {{'x' previously declared here}}
+  var x: Int // expected-error {{invalid redeclaration of 'x'}}
+  // expected-note@-1 {{cannot automatically synthesize 'Encodable' because '<<error type>>' does not conform to 'Encodable'}}
+  // expected-note@-2 {{cannot automatically synthesize 'Encodable' because '<<error type>>' does not conform to 'Encodable'}}
+}
+
+struct SR_12248_2: Decodable { 
+  var x: Int // expected-note {{'x' previously declared here}}
+  var x: Int // expected-error {{invalid redeclaration of 'x'}}
+}
+
+struct SR_12248_3: Encodable { 
+  var x: Int // expected-note {{'x' previously declared here}}
+  var x: Int // expected-error {{invalid redeclaration of 'x'}}
+}