as! and as? also indicate suppression of implicit existential opening

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -1478,9 +1478,9 @@ shouldOpenExistentialCallArgument(
   // An argument expression that explicitly coerces to an existential
   // disables the implicit opening of the existential.
   if (argExpr) {
-    if (auto argCoercion = dyn_cast<CoerceExpr>(
+    if (auto argCast = dyn_cast<ExplicitCastExpr>(
             argExpr->getSemanticsProvidingExpr())) {
-      if (auto typeRepr = argCoercion->getCastTypeRepr()) {
+      if (auto typeRepr = argCast->getCastTypeRepr()) {
         if (auto toType = cs.getType(typeRepr)) {
           if (toType->isAnyExistentialType())
             return None;

test/Constraints/opened_existentials_suppression.swift

@@ -1,13 +1,16 @@
 // RUN: %target-swift-frontend -enable-experimental-opened-existential-types -typecheck -dump-ast -parse-as-library %s | %FileCheck %s
 
 protocol P { }
+extension Optional: P where Wrapped: P { }
 
 func acceptsBox<T>(_ value: T) { }
 
 // CHECK: passBox
 // CHECK-NOT: open_existential_expr
 func passBox(p: P, obj: AnyObject, err: Error) {
   acceptsBox(p as P)
+  acceptsBox(p as! P)
+  acceptsBox(p as? P)
   acceptsBox(obj)
   acceptsBox(err)
 }