ConstantPropagation: fix dynamic cast from AnyHashable to a protocol.

rdar://problem/35364339

Author: eeckstein

lib/SIL/DynamicCasts.cpp

@@ -151,6 +151,11 @@ classifyDynamicCastToProtocol(CanType source,
     return DynamicCastFeasibility::WillFail;
   }
 
+  // AnyHashable is a special case: although it's a struct, there maybe another
+  // type conforming to it and to the TargetProtocol at the same time.
+  if (SourceNominalTy == SourceNominalTy->getASTContext().getAnyHashableDecl())
+    return DynamicCastFeasibility::MaySucceed;
+
   // If we are in a whole-module compilation and
   // if the source type is internal or target protocol is internal,
   // then conformances cannot be changed at run-time, because only this

test/SILOptimizer/anyhashable_to_protocol.swift

@@ -0,0 +1,18 @@
+// RUN: %target-swift-frontend -O -emit-sil %s | %FileCheck %s
+
+// REQUIRES: objc_interop
+
+import Foundation
+
+protocol FooProtocol : class {
+}
+
+class FooProtocolImplementation : NSObject, FooProtocol {
+}
+
+// CHECK-LABEL: sil @{{.*}}testit
+// CHECK: checked_cast_addr_br {{.*}} AnyHashable {{.*}} to FooProtocol 
+public func testit(_ x: AnyHashable) -> Bool {
+  return (x as? FooProtocol) != nil
+}
+