Add a test

test/SILOptimizer/devirtualize_protocol_composition.swift

@@ -0,0 +1,31 @@
+// RUN: %target-swift-frontend -parse-as-library -O -wmo -emit-sil  %s | %FileCheck %s
+// RUN: %target-swift-frontend -parse-as-library -Osize -wmo -emit-sil  %s | %FileCheck %s
+
+// This is an end-to-end test to ensure that the optimizer devertualizes
+// calls to a protocol composition type.
+
+public class ClassA<T> { }
+
+protocol ProtocolA {
+  func foo() -> Int
+}
+
+func shouldOptimizeWitness<T>(_ x: ClassA<T> & ProtocolA) -> Int {
+  return x.foo()
+}
+
+public class ClassB: ClassA<String> {
+  func foo() -> Int {
+    return 10
+  }
+}
+extension ClassB: ProtocolA { }
+
+//CHECK: witnessEntryPoint
+//CHECK-NOT: init_existential_ref
+//CHECK-NOT: open_existential_ref
+//CHECK-NOT: witness_method
+//CHECK: return
+public func witnessEntryPoint(c: ClassB) -> Int {
+  return shouldOptimizeWitness(c)
+}