[IRGen] Respect optionality of unowned(unsafe) reference properties

rdar://108705703

When generating the type info for fields marked unowned(unsafe) of an optional reference (e.g. AnyObject?), we dropped the fact that it was optional along the way. This caused incorrect tags to be returned when on object of the type was stored in an optional itself and the unowned property contained `nil`. In those cases the outer optional appeared to be `nil` as well, even if it in fact contained a value.

Author: drexin

lib/IRGen/GenExistential.cpp

@@ -798,6 +798,7 @@ namespace {
   class Name##ClassExistentialTypeInfo final \
     : public ScalarExistentialTypeInfoBase<Name##ClassExistentialTypeInfo, \
                                            LoadableTypeInfo> { \
+  bool IsOptional; \
   public: \
     Name##ClassExistentialTypeInfo( \
         ArrayRef<const ProtocolDecl *> storedProtocols, \
@@ -807,7 +808,8 @@ namespace {
         bool isOptional) \
       : ScalarExistentialTypeInfoBase(storedProtocols, ty, size, \
                                       spareBits, align, IsTriviallyDestroyable,\
-                                      IsCopyable, IsFixedSize) {} \
+                                      IsCopyable, IsFixedSize), \
+        IsOptional(isOptional) {} \
     TypeLayoutEntry \
     *buildTypeLayoutEntry(IRGenModule &IGM, \
                           SILType T, \
@@ -824,6 +826,18 @@ namespace {
       else \
         return IGM.getUnknownObjectTypeInfo(); \
     } \
+    unsigned getFixedExtraInhabitantCount(IRGenModule &IGM) const override { \
+      return getValueTypeInfoForExtraInhabitants(IGM) \
+                  .getFixedExtraInhabitantCount(IGM) - IsOptional; \
+    } \
+    APInt getFixedExtraInhabitantValue(IRGenModule &IGM, \
+                                       unsigned bits, \
+                                       unsigned index) const override { \
+      /* Note that we pass down the original bit-width. */ \
+      return getValueTypeInfoForExtraInhabitants(IGM) \
+                  .getFixedExtraInhabitantValue(IGM, bits, \
+                                                index + IsOptional); \
+    } \
     /* FIXME -- Use REF_STORAGE_HELPER and make */ \
     /* getValueTypeInfoForExtraInhabitants call llvm_unreachable() */ \
     void emitValueRetain(IRGenFunction &IGF, llvm::Value *value, \

test/Interpreter/rdar108801944.swift

@@ -0,0 +1,18 @@
+// RUN: %target-run-simple-swift | %FileCheck %s
+// REQUIRES: executable_test
+
+struct Context {
+    unowned(unsafe) var x: AnyObject? = nil
+}
+
+@inline(never)
+func test2(x: Context) -> Context? {
+  return .some(x)
+}
+
+// CHECK: works
+if (test2(x: Context()) == nil) {
+    print("bug")
+} else {
+    print("works")
+}