Allow inheritance from resilient classes with missing members

When a class has missing vtable entries, we don’t allow it to be subclassed. This is unnecessarily restrictive for resilient classes, and @_implementationOnly imports now make missing vtable entries much more common. This commit carves out an exception to that rule for resilient classes.

Fixes <rdar://problem/50902125>.

Author: beccadax

Diff for: lib/Sema/TypeCheckDecl.cpp

@@ -2958,7 +2958,9 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
         break;
       }
 
-      if (!isInvalidSuperclass && Super->hasMissingVTableEntries()) {
+      if (!isInvalidSuperclass && Super->hasMissingVTableEntries() &&
+          !Super->isResilient(CD->getParentModule(),
+                              ResilienceExpansion::Minimal)) {
         auto *superFile = Super->getModuleScopeContext();
         if (auto *serialized = dyn_cast<SerializedASTFile>(superFile)) {
           if (serialized->getLanguageVersionBuiltWith() !=

Diff for: test/decl/class/override-implementationOnly.swift

@@ -0,0 +1,52 @@
+// RUN: %empty-directory(%t)
+
+// For convenience, this file includes the three different "files" used in this
+// test. It selects one with -DCoreDishwasher, -DDishwasherKit, or neither.
+
+// RUN: %target-swift-frontend -emit-module %s -DCoreDishwasher -module-name CoreDishwasher -o %t/CoreDishwasher -emit-module-path %t/CoreDishwasher.swiftmodule -I %t
+// RUN: %target-swift-frontend -emit-module %s -DDishwasherKit -module-name DishwasherKit -o %t/DishwasherKit -emit-module-path %t/DishwasherKit.swiftmodule -enable-library-evolution -I %t
+// RUN: %target-typecheck-verify-swift -I %t
+
+#if CoreDishwasher
+
+  public struct SpimsterWicket {
+    public init() {}
+  }
+
+#elseif DishwasherKit
+
+  @_implementationOnly import CoreDishwasher
+
+  open class Dishwasher {
+    public init() {}
+    
+    var wicket = SpimsterWicket()
+    
+    open var modelName: String { "Dishwasher" }
+  }
+
+  open class NoUserServiceablePartsInside {
+    public init() {}
+    internal init(wicket: SpimsterWicket) {}
+    public convenience init(inconvenient: ()) {
+      self.init()
+    }
+  }
+
+#else
+
+  import DishwasherKit
+
+  class FancyDishwasher: Dishwasher {
+    override var modelName: String { "Fancy \(super.modelName)" }
+  }
+
+  class VoidedWarranty: NoUserServiceablePartsInside {
+    override init() { super.init() }
+  }
+
+  // FIXME: This diagnostic should be better, but it matches what we're already
+  // doing for disallowed convenience inits.
+  let notAllowed = VoidedWarranty(inconvenient: ()) // expected-error{{argument passed to call that takes no arguments}}
+
+#endif