Add tests for multiple @_implements on associated types

Author: DougGregor

test/decl/protocol/req/associated_type_typealias_implements.swift

@@ -36,3 +36,33 @@ struct Y3: Q { // expected-error{{type 'Y3' does not conform to protocol 'Q'}}
   typealias T = XT
   // FIXME: More detail from diagnostic.
 }
+
+
+protocol A1 {
+  associatedtype T
+}
+protocol A2 {
+  associatedtype T
+}
+
+struct BothA1_and_A2: A1, A2 {
+  @_implements(A1, T)
+  typealias X = Int
+
+  @_implements(A2, T)
+  typealias Y = String
+}
+
+typealias A1_T<U: A1> = U.T
+typealias A2_T<U: A2> = U.T
+
+struct RequireSame<T, U> { }
+
+extension RequireSame where T == U {
+  init(same: Bool) { }
+}
+
+func testImplements() {
+  _ = RequireSame<A1_T<BothA1_and_A2>, Int>(same: true)
+  _ = RequireSame<A2_T<BothA1_and_A2>, String>(same: true)
+}