Fix OSSA in SILGenFunction::emitIVarDestroyer

It is illegal to borrow an unowned value. Unowned values are only
allowed to have specific instantaneous uses.

In this case, an unchecked conversion is valid because there is an
assumption that within a destructor, self is guaranteed.

Author: atrick

lib/SILGen/SILGenBuilder.h

@@ -272,7 +272,7 @@ class SILGenBuilder : public SILBuilder {
                                              SILType type);
 
   using SILBuilder::createUncheckedRefCast;
-  ManagedValue createUncheckedRefCast(SILLocation loc, ManagedValue original,
+  ManagedValue createUncheckedRefCast(SILLocation loc, ManagedValue value,
                                       SILType type);
 
   using SILBuilder::createUncheckedAddrCast;

lib/SILGen/SILGenDestructor.cpp

@@ -169,6 +169,17 @@ void SILGenFunction::emitIVarDestroyer(SILDeclRef ivarDestroyer) {
   prepareEpilog(false, false, cleanupLoc);
   {
     Scope S(*this, cleanupLoc);
+    // Self is effectively guaranteed for the duration of any destructor.  For
+    // ObjC classes, self may be unowned. A conversion to guaranteed is required
+    // to access its members.
+    if (selfValue.getOwnershipKind() != OwnershipKind::Guaranteed) {
+      // %guaranteedSelf = unchecked_ownership_conversion %self to @guaranteed
+      // ...
+      // end_borrow %guaranteedSelf
+      auto guaranteedSelf = B.createUncheckedOwnershipConversion(
+        cleanupLoc, selfValue.forward(*this), OwnershipKind::Guaranteed);
+      selfValue = emitManagedBorrowedRValueWithCleanup(guaranteedSelf);
+    }
     emitClassMemberDestruction(selfValue, cd, cleanupLoc);
   }
 
@@ -179,7 +190,7 @@ void SILGenFunction::emitIVarDestroyer(SILDeclRef ivarDestroyer) {
 void SILGenFunction::emitClassMemberDestruction(ManagedValue selfValue,
                                                 ClassDecl *cd,
                                                 CleanupLocation cleanupLoc) {
-  selfValue = selfValue.borrow(*this, cleanupLoc);
+  assert(selfValue.getOwnershipKind() == OwnershipKind::Guaranteed);
   for (VarDecl *vd : cd->getStoredProperties()) {
     const TypeLowering &ti = getTypeLowering(vd->getType());
     if (!ti.isTrivial()) {

test/SILGen/ivar_destroyer_objc.swift

@@ -0,0 +1,18 @@
+// RUN: %target-swift-emit-silgen(mock-sdk: %clang-importer-sdk) %s | %FileCheck %s
+// REQUIRES: objc_interop
+
+import Foundation
+
+// CHECK-LABEL: sil hidden [ossa] @$s19ivar_destroyer_objc7ObjCFooCfETo : $@convention(objc_method) (ObjCFoo) -> () {
+// CHECK: bb0(%0 : @unowned $ObjCFoo):
+// CHECK:   [[CONVERT:%.*]] = unchecked_ownership_conversion %0 : $ObjCFoo, @unowned to @guaranteed
+// CHECK:   ref_element_addr [[CONVERT]] : $ObjCFoo, #ObjCFoo.object
+// CHECK:   begin_access [deinit] [static]
+// CHECK:   destroy_addr
+// CHECK:   end_access
+// CHECK:   end_borrow [[CONVERT]] : $ObjCFoo
+// CHECK-LABEL: } // end sil function '$s19ivar_destroyer_objc7ObjCFooCfETo'
+internal class ObjCFoo : NSObject {
+  var object: AnyObject
+  init(o: AnyObject) { object = o }
+}