Index: lldb/packages/Python/lldbsuite/test/lang/objc/modules-app-update/Makefile
===================================================================
--- /dev/null
+++ lldb/packages/Python/lldbsuite/test/lang/objc/modules-app-update/Makefile
@@ -0,0 +1,5 @@
+CFLAGS_EXTRAS = -I$(BUILDDIR)
+USE_PRIVATE_MODULE_CACHE := YES
+OBJC_SOURCES := main.m foo.m
+
+include Makefile.rules
Index: lldb/packages/Python/lldbsuite/test/lang/objc/modules-app-update/TestClangModulesAppUpdate.py
===================================================================
--- /dev/null
+++ lldb/packages/Python/lldbsuite/test/lang/objc/modules-app-update/TestClangModulesAppUpdate.py
@@ -0,0 +1,61 @@
+from __future__ import print_function
+
+import unittest2
+import os
+import shutil
+
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+
+class TestClangModuleAppUpdate(TestBase):
+    mydir = TestBase.compute_mydir(__file__)
+
+    def setUp(self):
+        TestBase.setUp(self)
+
+    @skipUnlessDarwin
+    @skipIf(debug_info=no_match(["gmodules"]))
+    def test_rebuild_app_modules_untouched(self):
+        with open(self.getBuildArtifact("module.modulemap"), "w") as f:
+            f.write("""
+                    module Foo { header "f.h" }
+                    """)
+        with open(self.getBuildArtifact("f.h"), "w") as f:
+            f.write("""
+                    @import Foundation;
+                    @interface Foo : NSObject {
+                       int i;
+                    }
+                    +(instancetype)init;
+                    @end
+                    """)
+
+        mod_cache = self.getBuildArtifact("private-module-cache")
+        import os
+        if os.path.isdir(mod_cache):
+          shutil.rmtree(mod_cache)
+        self.build()
+        self.assertTrue(os.path.isdir(mod_cache), "module cache exists")
+
+        target, process, _, bkpt = lldbutil.run_to_source_breakpoint(
+            self, "break here", lldb.SBFileSpec("main.m"))
+        bar = target.FindTypes('Bar').GetTypeAtIndex(0)
+        foo = bar.GetDirectBaseClassAtIndex(0).GetType()
+        self.assertEqual(foo.GetNumberOfFields(), 1)
+        self.assertEqual(foo.GetFieldAtIndex(0).GetName(), "i")
+
+        # Rebuild.
+        process.Kill()
+        os.remove(self.getBuildArtifact('main.o'))
+        os.remove(self.getBuildArtifact('a.out'))
+        self.build()
+
+        # Reattach.
+        target, process, _, _ = lldbutil.run_to_breakpoint_do_run(self, target, bkpt)
+        bar = target.FindTypes('Bar').GetTypeAtIndex(0)
+        foo = bar.GetDirectBaseClassAtIndex(0).GetType()
+        self.assertEqual(foo.GetNumberOfFields(), 1)
+        self.assertEqual(foo.GetFieldAtIndex(0).GetName(), "i")
Index: lldb/packages/Python/lldbsuite/test/lang/objc/modules-app-update/foo.m
===================================================================
--- /dev/null
+++ lldb/packages/Python/lldbsuite/test/lang/objc/modules-app-update/foo.m
@@ -0,0 +1,7 @@
+@import Foundation;
+@import Foo;
+@implementation Foo
++(instancetype)init {
+  return [super init];
+}
+@end
Index: lldb/packages/Python/lldbsuite/test/lang/objc/modules-app-update/main.m
===================================================================
--- /dev/null
+++ lldb/packages/Python/lldbsuite/test/lang/objc/modules-app-update/main.m
@@ -0,0 +1,17 @@
+@import Umbrella;
+
+@interface Bar : Foo
++(instancetype)init;
+@end
+
+@implementation Bar
++(instancetype)init {
+  return [super init];
+}
+@end
+
+int main(int argc, char **argv) {
+  id bar = [Bar new];
+  [bar i]; // break here
+  return 0;
+}
Index: lldb/packages/Python/lldbsuite/test/lang/objc/modules-app-update/module.modulemap
===================================================================
--- /dev/null
+++ lldb/packages/Python/lldbsuite/test/lang/objc/modules-app-update/module.modulemap
@@ -0,0 +1,4 @@
+module Umbrella {
+  header "umbrella.h"
+  export *
+}
Index: lldb/packages/Python/lldbsuite/test/lang/objc/modules-app-update/umbrella.h
===================================================================
--- /dev/null
+++ lldb/packages/Python/lldbsuite/test/lang/objc/modules-app-update/umbrella.h
@@ -0,0 +1 @@
+@import Foo;
Index: lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp
===================================================================
--- lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp
+++ lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp
@@ -187,7 +187,7 @@
   languages.Insert(die.GetCU()->GetLanguageType());
   llvm::DenseSet<SymbolFile *> searched_symbol_files;
   clang_module_sp->GetSymbolFile()->FindTypes(decl_context, languages,
-                                            searched_symbol_files, pcm_types);
+                                              searched_symbol_files, pcm_types);
   if (pcm_types.Empty()) {
     // Since this type is defined in one of the Clang modules imported
     // by this symbol file, search all of them. Instead of calling
@@ -222,6 +222,19 @@
   if (!type)
     return TypeSP();
 
+  // Under normal operation pcm_type is a shallow forward declaration
+  // that gets completed later. This is necessary to support cyclic
+  // data structures. If, however, pcm_type is already complete (for
+  // example, because it was loaded for a different target before),
+  // the definition needs to be imported right away, too.
+  // Type::ResolveClangType() effectively ignores the ResolveState
+  // inside type_sp and only looks at IsDefined(), so it never calls
+  // ClangASTImporter::ASTImporterDelegate::ImportDefinitionTo(),
+  // which does extra work for Objective-C classes. This would result
+  // in only the forward declaration to be visible.
+  if (pcm_type.IsDefined())
+    GetClangASTImporter().RequireCompleteType(ClangUtil::GetQualType(type));
+
   SymbolFileDWARF *dwarf = die.GetDWARF();
   TypeSP type_sp(new Type(
       die.GetID(), dwarf, pcm_type_sp->GetName(), pcm_type_sp->GetByteSize(),