Sema: Fix for derived classes where the base class has inaccessible initializers

If the base class is in a different module and defines an internal
initializer, or it is in a different file and defines a private
initializer, the subclass cannot access this initializer.

Fix this by insisting on synthesizing initializers even if they
override an internal or private initializer in the base class.
In this case, synthesize them as a stub instead of a chaining
initializer. They cannot be called, and they cannot reference
the superclass initializer, so a stub is appropriate here.

Also in SILGen, emit vtable entries for stub initializers unless
they are overriding an Objective-C initializer.

Author: slavapestov

lib/SILGen/SILGenType.cpp

@@ -243,8 +243,9 @@ class SILGenVTable : public Lowering::ASTVisitor<SILGenVTable> {
   }
 
   void visitConstructorDecl(ConstructorDecl *cd) {
-    // Stub constructors don't get an entry.
-    if (cd->hasStubImplementation())
+    // Stub constructors don't get an entry, unless they were synthesized to
+    // override a non-required designated initializer in the superclass.
+    if (cd->hasStubImplementation() && !cd->getOverriddenDecl())
       return;
 
     // Required constructors (or overrides thereof) have their allocating entry

lib/Sema/CodeSynthesis.cpp

@@ -2155,7 +2155,7 @@ swift::createDesignatedInitOverride(TypeChecker &tc,
 
   // Wire up the overrides.
   ctor->getAttrs().add(new (tc.Context) OverrideAttr(/*Implicit=*/true));
-  checkOverrides(tc, ctor);
+  ctor->setOverriddenDecl(superclassCtor);
 
   if (kind == DesignatedInitKind::Stub) {
     // Make this a stub implementation.

lib/Sema/TypeCheckDecl.cpp

@@ -7547,7 +7547,10 @@ void TypeChecker::addImplicitConstructors(NominalTypeDecl *decl) {
     }
 
     auto superclassTy = classDecl->getSuperclass();
-    for (auto memberResult : lookupConstructors(classDecl, superclassTy)) {
+    auto ctors = lookupConstructors(classDecl, superclassTy,
+                                    NameLookupFlags::IgnoreAccessibility);
+
+    for (auto memberResult : ctors) {
       auto member = memberResult.Decl;
 
       // Skip unavailable superclass initializers.
@@ -7595,12 +7598,23 @@ void TypeChecker::addImplicitConstructors(NominalTypeDecl *decl) {
              getInitializerParamType(superclassCtor)).second)
         continue;
 
+      // If we're inheriting initializers, create an override delegating
+      // to 'super.init'. Otherwise, create a stub which traps at runtime.
+      auto kind = canInheritInitializers
+                    ? DesignatedInitKind::Chaining
+                    : DesignatedInitKind::Stub;
+
+      // If the superclass initializer is not accessible from the derived
+      // class, we cannot chain to 'super.init' either -- create a stub.
+      if (!superclassCtor->isAccessibleFrom(classDecl)) {
+        assert(!superclassCtor->isRequired() &&
+               "required initializer less visible than the class?");
+        kind = DesignatedInitKind::Stub;
+      }
+
       // We have a designated initializer. Create an override of it.
       if (auto ctor = createDesignatedInitOverride(
-                        *this, classDecl, superclassCtor,
-                        canInheritInitializers
-                          ? DesignatedInitKind::Chaining
-                          : DesignatedInitKind::Stub)) {
+                        *this, classDecl, superclassCtor, kind)) {
         classDecl->addMember(ctor);
       }
     }

test/SILGen/accessibility_vtables.swift

@@ -12,12 +12,16 @@ class Sub : Base {
   }
 }
 
+// CHECK-LABEL: sil hidden @_TFC21accessibility_vtables3SubcfT_S0_ : $@convention(method) (@owned Sub) -> @owned Sub
+// CHECK:       bb0(%0 : $Sub):
+// CHECK:         function_ref @_TFs26_unimplemented_initializerFT9classNameVs12StaticString8initNameS_4fileS_4lineSu6columnSu_T_
+
 // CHECK-LABEL: sil_vtable Sub {
 // CHECK-NEXT:  #Base.internalMethod!1: _TFC28accessibility_vtables_helper4Base14internalMethod
 // CHECK-NEXT:  #Base.prop!getter.1: _TFC21accessibility_vtables3Subg4propSi  // accessibility_vtables.Sub.prop.getter : Swift.Int
 // CHECK-NEXT:  #Base.prop!setter.1: _TFC28accessibility_vtables_helper4Bases4propSi  // accessibility_vtables_helper.Base.prop.setter : Swift.Int
 // CHECK-NEXT:  #Base.prop!materializeForSet.1: _TFC28accessibility_vtables_helper4Basem4propSi  // accessibility_vtables_helper.Base.prop.materializeForSet : Swift.Int
-// CHECK-NEXT:  #Base.init!initializer.1: _TFC28accessibility_vtables_helper4Basec
+// CHECK-NEXT:  #Base.init!initializer.1: _TFC21accessibility_vtables3SubcfT_S0_  // accessibility_vtables.Sub.init () -> accessibility_vtables.Sub
 // CHECK-NEXT: #Sub.internalMethod!1: _TFC21accessibility_vtables3Sub14internalMethod
 // CHECK-NEXT: #Sub.prop!setter.1: _TFC21accessibility_vtables3Subs4propSi   // accessibility_vtables.Sub.prop.setter : Swift.Int
 // CHECK-NEXT: #Sub.prop!materializeForSet.1: _TFC21accessibility_vtables3Subm4propSi  // accessibility_vtables.Sub.prop.materializeForSet : Swift.Int