[Runtime] Let existential types satisfy superclass requirements.

Previously, when an attempt was made to instantiate a generic metadata
whose argument was constrained to subclass a superclass Super with an
existential type E that had a superclass constraint to a subclass Sub of
that same superclass Super, the instantiation would fail at runtime,
despite the fact that the generic instantiation was allowed to
type-check.

The result was a runtime failure to instantiate generic metadata
resulting eventually in a crash.

Here, handling for that situation is added.  When checking generic
requirements at runtime, when a superclass requirement is encountered,
an existential type is checked for.  If that existential type has a
superclass constraint and if that superclass constraint is a subclass of
the superclass requirement, the check is determined to be satisfactory.

rdar://problem/64672291

Author: nate-chandler

stdlib/public/runtime/ProtocolConformance.cpp

@@ -841,6 +841,15 @@ bool swift::_checkGenericRequirements(
                                    substGenericParam, substWitnessTable).getMetadata();
       if (!baseType) return true;
 
+      // If the type which is constrained to a base class is an existential 
+      // type, and if that existential type includes a superclass constraint,
+      // just require that the superclass by which the existential is
+      // constrained is a subclass of the base class.
+      if (auto *existential = dyn_cast<ExistentialTypeMetadata>(subjectType)) {
+        if (auto *superclassConstraint = existential->getSuperclassConstraint())
+          subjectType = superclassConstraint;
+      }
+
       if (!isSubclass(subjectType, baseType))
         return true;
 

validation-test/Runtime/rdar64672291.swift

@@ -0,0 +1,24 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: objc_interop
+
+import Foundation
+
+final class Storage<T: NSObject> {
+    weak var object: T?
+    init(object: T) {
+        self.object = object
+    }
+}
+
+
+@objc protocol MyProtocol {}
+typealias MyStorage = Storage<NSObject & MyProtocol>
+
+class Gadget: NSObject, MyProtocol {
+    func testit() {
+        _ = MyStorage(object: self)
+    }
+}
+
+let gadget = Gadget()
+gadget.testit()