[PrintAsObjC] Attempt at fixing SR-5018: marking +new unavailable when -init is unavailable.

Currently a loop-hole exists by which one could end up invoking an unavailable -init initializer from Obj-C
when it is unavailable by using +new (which itself calls -init)

https://bugs.swift.org/browse/SR-5018

lib/PrintAsObjC/PrintAsObjC.cpp

@@ -604,6 +604,7 @@ private:
     }
 
     bool skipAvailability = false;
+    bool makeNewUnavailable = false;
     // Swift designated initializers are Objective-C designated initializers.
     if (auto ctor = dyn_cast<ConstructorDecl>(AFD)) {
       if (ctor->hasStubImplementation()
@@ -612,6 +613,9 @@ private:
         // required access
         os << " SWIFT_UNAVAILABLE";
         skipAvailability = true;
+        // If -init is unavailable, then +new should be, too:
+        const bool selectorIsInit = selectorPieces.getNumArgs() == 0 && selectorPieces.front().str() == "init";
+        makeNewUnavailable = selectorIsInit;
       } else if (ctor->isDesignatedInit() &&
           !isa<ProtocolDecl>(ctor->getDeclContext())) {
         os << " OBJC_DESIGNATED_INITIALIZER";
@@ -643,6 +647,10 @@ private:
     }
 
     os << ";\n";
+
+    if (makeNewUnavailable) {
+        os << "+ (nonnull instancetype)new SWIFT_UNAVAILABLE;\n";
+    }
   }
 
   void printAbstractFunctionAsFunction(FuncDecl *FD) {

test/PrintAsObjC/availability.swift

@@ -57,6 +57,7 @@
 
 // CHECK-LABEL: @interface AvailabilitySub
 // CHECK-NEXT: - (nonnull instancetype)init SWIFT_UNAVAILABLE;
+// CHECK-NEXT: + (nonnull instancetype)new SWIFT_UNAVAILABLE;
 // CHECK-NEXT: - (nonnull instancetype)initWithX:(NSInteger)_ SWIFT_UNAVAILABLE;
 // CHECK-NEXT: @end
 

test/PrintAsObjC/classes.swift

@@ -175,6 +175,15 @@ class DiscardableResult : NSObject {
   @objc init(evenMoreFun: ()) { super.init() }
 }
 
+// CHECK-LABEL: @interface InheritedInitializersRequired
+// CHECK-NEXT: - (nonnull instancetype)initWithEvenMoreFun OBJC_DESIGNATED_INITIALIZER;
+// CHECK-NEXT: - (nonnull instancetype)init SWIFT_UNAVAILABLE;
+// CHECK-NEXT: + (nonnull instancetype)new SWIFT_UNAVAILABLE;
+// CHECK-NEXT: @end
+@objc class InheritedInitializersRequired : InheritedInitializers {
+  @objc required init(evenMoreFun: ()) { super.init() }
+}
+
 // NEGATIVE-NOT: NotObjC
 class NotObjC {}
 

test/PrintAsObjC/protocols.swift

@@ -55,6 +55,7 @@ protocol CustomNameType2 {}
 // CHECK-LABEL: @interface MyObject : NSObject <NSCoding, Fungible>
 // CHECK-NEXT: initWithCoder
 // CHECK-NEXT: init SWIFT_UNAVAILABLE
+// CHECK-NEXT: new SWIFT_UNAVAILABLE
 // CHECK-NEXT: @end
 // NEGATIVE-NOT: @protocol NSCoding
 class MyObject : NSObject, NSCoding, Fungible {