diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -8474,18 +8474,20 @@
       continue;
     }
 
-    NamedDecl *D = cast<NamedDecl>(GetDecl(DeclIDs[I]));
+    if (Decl *DeserializedD = GetDecl(DeclIDs[I])) {
+      NamedDecl *D = cast<NamedDecl>(DeserializedD);
 
-    // If we're simply supposed to record the declarations, do so now.
-    if (Decls) {
-      Decls->push_back(D);
-      continue;
-    }
+      // If we're simply supposed to record the declarations, do so now.
+      if (Decls) {
+        Decls->push_back(D);
+        continue;
+      }
 
-    // Introduce this declaration into the translation-unit scope
-    // and add it to the declaration chain for this identifier, so
-    // that (unqualified) name lookup will find it.
-    pushExternalDeclIntoScope(D, II);
+      // Introduce this declaration into the translation-unit scope
+      // and add it to the declaration chain for this identifier, so
+      // that (unqualified) name lookup will find it.
+      pushExternalDeclIntoScope(D, II);
+    }
   }
 }
 
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -3870,6 +3870,15 @@
     llvm::report_fatal_error(
         Twine("ASTReader::readDeclRecord failed reading decl code: ") +
         toString(MaybeDeclCode.takeError()));
+  if (unsigned DeclSubmoduleID = getGlobalSubmoduleID(*Loc.F, Record.readInt())) {
+    if (SemaObj) {
+      Module *DeclOwner = getSubmodule(DeclSubmoduleID);
+      if (DeclOwner && !SemaObj->isModuleVisible(DeclOwner)) {
+        // Skip deserialization of Decls from a hidden module.
+        return nullptr;
+      }
+    }
+  }
   switch ((DeclCode)MaybeDeclCode.get()) {
   case DECL_CONTEXT_LEXICAL:
   case DECL_CONTEXT_VISIBLE:
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -269,6 +269,7 @@
 }
 
 void ASTDeclWriter::Visit(Decl *D) {
+  Record.push_back(Writer.getSubmoduleID(D->getOwningModule()));
   DeclVisitor<ASTDeclWriter>::Visit(D);
 
   // Source locations require array (variable-length) abbreviations.  The
@@ -1911,6 +1912,7 @@
   // Abbreviation for DECL_FIELD
   Abv = std::make_shared<BitCodeAbbrev>();
   Abv->Add(BitCodeAbbrevOp(serialization::DECL_FIELD));
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // Decl
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // DeclContext
   Abv->Add(BitCodeAbbrevOp(0));                       // LexicalDeclContext
@@ -1944,6 +1946,7 @@
   // Abbreviation for DECL_OBJC_IVAR
   Abv = std::make_shared<BitCodeAbbrev>();
   Abv->Add(BitCodeAbbrevOp(serialization::DECL_OBJC_IVAR));
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // Decl
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // DeclContext
   Abv->Add(BitCodeAbbrevOp(0));                       // LexicalDeclContext
@@ -1980,6 +1983,7 @@
   // Abbreviation for DECL_ENUM
   Abv = std::make_shared<BitCodeAbbrev>();
   Abv->Add(BitCodeAbbrevOp(serialization::DECL_ENUM));
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // Redeclarable
   Abv->Add(BitCodeAbbrevOp(0));                       // No redeclaration
   // Decl
@@ -2030,6 +2034,7 @@
   // Abbreviation for DECL_RECORD
   Abv = std::make_shared<BitCodeAbbrev>();
   Abv->Add(BitCodeAbbrevOp(serialization::DECL_RECORD));
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // Redeclarable
   Abv->Add(BitCodeAbbrevOp(0));                       // No redeclaration
   // Decl
@@ -2092,6 +2097,7 @@
   // Abbreviation for DECL_PARM_VAR
   Abv = std::make_shared<BitCodeAbbrev>();
   Abv->Add(BitCodeAbbrevOp(serialization::DECL_PARM_VAR));
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // Redeclarable
   Abv->Add(BitCodeAbbrevOp(0));                       // No redeclaration
   // Decl
@@ -2140,6 +2146,7 @@
   // Abbreviation for DECL_TYPEDEF
   Abv = std::make_shared<BitCodeAbbrev>();
   Abv->Add(BitCodeAbbrevOp(serialization::DECL_TYPEDEF));
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // Redeclarable
   Abv->Add(BitCodeAbbrevOp(0));                       // No redeclaration
   // Decl
@@ -2169,6 +2176,7 @@
   // Abbreviation for DECL_VAR
   Abv = std::make_shared<BitCodeAbbrev>();
   Abv->Add(BitCodeAbbrevOp(serialization::DECL_VAR));
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // Redeclarable
   Abv->Add(BitCodeAbbrevOp(0));                       // No redeclaration
   // Decl
@@ -2221,6 +2229,7 @@
   // Abbreviation for DECL_CXX_METHOD
   Abv = std::make_shared<BitCodeAbbrev>();
   Abv->Add(BitCodeAbbrevOp(serialization::DECL_CXX_METHOD));
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // RedeclarableDecl
   Abv->Add(BitCodeAbbrevOp(0));                         // CanonicalDecl
   // Decl
diff --git a/clang/test/Modules/ambiguous-enum-lookup.m b/clang/test/Modules/ambiguous-enum-lookup.m
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/ambiguous-enum-lookup.m
@@ -0,0 +1,42 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: %clang_cc1 -fsyntax-only -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/modules.cache -F %t/Frameworks %t/test.m
+
+//--- Frameworks/NonModular.framework/Headers/NonModular.h
+#ifndef NonModular_NonModular_h
+#define NonModular_NonModular_h
+struct SomeStruct {
+    int x;
+};
+
+enum SomeEnum {
+    kEnumValue = 1,
+};
+#endif
+
+//--- Frameworks/PiecewiseVisible.framework/Headers/InitiallyVisible.h
+// empty
+
+//--- Frameworks/PiecewiseVisible.framework/Headers/InitiallyHidden.h
+#include <NonModular/NonModular.h>
+
+//--- Frameworks/PiecewiseVisible.framework/Modules/module.modulemap
+framework module PiecewiseVisible {
+  header "InitiallyVisible.h"
+  export *
+
+  explicit module HiddenPiece {
+    header "InitiallyHidden.h"
+    export *
+  }
+}
+
+//--- test.m
+#include <PiecewiseVisible/InitiallyVisible.h>
+#include <NonModular/NonModular.h>
+#include <PiecewiseVisible/InitiallyHidden.h>
+
+void test() {
+    struct SomeStruct s;
+    s.x = kEnumValue;
+}