Sema: Targeted fix for LLDB REPL issue

LLDB changes accessiblity of declarations after type checking,
which is not a good idea because it is likely to break invariants.
Indeed, the validateFixedLayoutAttribute() / hasFixedLayout()
logic was not prepared for this possibility.

This is a targeted fix to address the immediate breakage. A better
fix would be to change LLDB, and also to change Sema to store the
global -enable-resilience flag state in a bit in the serialized
module, instead of sticking it on every declaration.

Fixes <rdar://problem/23545959>.

lib/AST/Decl.cpp

@@ -1857,6 +1857,10 @@ bool NominalTypeDecl::hasFixedLayout() const {
     return true;
   }
 
+  // Objective-C enums always have a fixed layout.
+  if (isa<EnumDecl>(this) && isObjC())
+    return true;
+
   // Otherwise, access via indirect "resilient" interfaces.
   return false;
 }

lib/Sema/TypeCheckDecl.cpp

@@ -6957,18 +6957,17 @@ static void validateFixedLayoutAttribute(TypeChecker &TC,
                                          NominalTypeDecl *D) {
   DeclAttributes &Attrs = D->getAttrs();
 
-  if (Attrs.hasAttribute<FixedLayoutAttr>())
+  // FIXME: Add a per-module serialized HasFixedLayout flag, instead of
+  // giving every decl this attribute.
+
+  if (Attrs.hasAttribute<FixedLayoutAttr>() ||
+      TC.Context.LangOpts.EnableResilience)
     return;
 
   // Since -enable-resilience should not change how we call into
   // existing compiled modules, make all value types @_fixed_layout
   // when the frontend is not run with the -enable-resilience flag.
-  if (!TC.Context.LangOpts.EnableResilience &&
-      D->getFormalAccess() == Accessibility::Public)
-    Attrs.add(new (TC.Context) FixedLayoutAttr(/*IsImplicit*/ true));
-  // @objc enums are always @_fixed_layout.
-  else if (isa<EnumDecl>(D) && D->isObjC())
-    Attrs.add(new (TC.Context) FixedLayoutAttr(/*IsImplicit*/ true));
+  Attrs.add(new (TC.Context) FixedLayoutAttr(/*IsImplicit*/ true));
 }
 
 static void validateAttributes(TypeChecker &TC, Decl *D) {

test/Serialization/alignment.swift

@@ -3,8 +3,8 @@
 // RUN: %target-swift-frontend %s -emit-module -parse-as-library -o %t
 // RUN: %target-sil-opt -enable-sil-verify-all %t/alignment.swiftmodule -o - | FileCheck %s
 
-//CHECK: @_alignment(16) struct Foo {
-@_alignment(16) struct Foo {}
+//CHECK: @_alignment(16) @_fixed_layout struct Foo {
+@_alignment(16) @_fixed_layout struct Foo {}
 
 func foo(x: Foo) {}