Restrict implicit opening of existentials based on the generic function's requirements

Rather than looking solely at the existential argument of a generic
function to determine whether it self-conforms, compare the
existential argument against each of the protocols to which the
corresponding generic parameter conforms. This better models when the
existential value itself will successfully meet the requirements of
the generic function, and therefore not require opening.

This provides far better source compatibility by not changing
semantics of existing, well-formed calls, and eliminates the need to
special-case CoW-related operations in the standard library.

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -1501,32 +1501,6 @@ shouldOpenExistentialCallArgument(
   if (!argTy->isAnyExistentialType())
     return None;
 
-
-  // If the existential argument type conforms to all of its protocol
-  // requirements, don't open the existential.
-  {
-    Type existentialObjectType;
-    if (auto existentialMetaTy = argTy->getAs<ExistentialMetatypeType>())
-      existentialObjectType = existentialMetaTy->getInstanceType();
-    else
-      existentialObjectType = argTy;
-    auto layout = existentialObjectType->getExistentialLayout();
-    auto module = cs.DC->getParentModule();
-    bool containsNonSelfConformance = false;
-    for (auto proto : layout.getProtocols()) {
-      auto protoDecl = proto->getDecl();
-      auto conformance = module->lookupExistentialConformance(
-          existentialObjectType, protoDecl);
-      if (conformance.isInvalid()) {
-        containsNonSelfConformance = true;
-        break;
-      }
-    }
-
-    if (!containsNonSelfConformance)
-      return None;
-  }
-
   auto param = getParameterAt(callee, paramIdx);
   if (!param)
     return None;
@@ -1568,6 +1542,31 @@ shouldOpenExistentialCallArgument(
           genericSig.getGenericParams().back()->getDepth())
     return None;
 
+  // If the existential argument conforms to all of protocol requirements on
+  // the formal parameter's type, don't open.
+  // If all of the conformance requirements on the formal parameter's type
+  // are self-conforming, don't open.
+  {
+    Type existentialObjectType;
+    if (auto existentialMetaTy = argTy->getAs<ExistentialMetatypeType>())
+      existentialObjectType = existentialMetaTy->getInstanceType();
+    else
+      existentialObjectType = argTy;
+    auto module = cs.DC->getParentModule();
+    bool containsNonSelfConformance = false;
+    for (auto proto : genericSig->getRequiredProtocols(genericParam)) {
+      auto conformance = module->lookupExistentialConformance(
+          existentialObjectType, proto);
+      if (conformance.isInvalid()) {
+        containsNonSelfConformance = true;
+        break;
+      }
+    }
+
+    if (!containsNonSelfConformance)
+      return None;
+  }
+
   // Ensure that the formal parameter is only used in covariant positions,
   // because it won't match anywhere else.
   auto referenceInfo = findGenericParameterReferences(

test/Constraints/opened_existentials_suppression.swift

@@ -9,6 +9,7 @@ func acceptsBoxType<T>(_ value: T.Type) { }
 // CHECK: passBox
 // CHECK-NOT: open_existential_expr
 func passBox(p: P, obj: AnyObject, err: Error) {
+  acceptsBox(p)
   acceptsBox(p as P)
   acceptsBox(p as! P)
   acceptsBox(p as? P)