Index: lib/Serialization/ASTReaderDecl.cpp
===================================================================
--- lib/Serialization/ASTReaderDecl.cpp
+++ lib/Serialization/ASTReaderDecl.cpp
@@ -2541,6 +2541,20 @@
   }
 }
 
+/// ODR-like semantics for C/ObjC allow us to merge tag types and a structural
+/// check in Sema guarantees the types can be merged (see C11 6.2.7/1 or C89
+/// 6.1.2.6/1). Although most merging is done in Sema, we need to guarantee
+/// that some types are mergeable during deserialization, otherwise name
+/// lookup fails. This is the case for EnumConstantDecl.
+bool allowODRLikeMergeInC(NamedDecl *ND) {
+  if (!ND)
+    return false;
+  // TODO: implement merge for other necessary decls.
+  if (dyn_cast<EnumConstantDecl>(ND))
+    return true;
+  return false;
+}
+
 /// \brief Attempts to merge the given declaration (D) with another declaration
 /// of the same entity, for the case where the entity is not actually
 /// redeclarable. This happens, for instance, when merging the fields of
@@ -2553,8 +2567,10 @@
 
   // ODR-based merging is only performed in C++. In C, identically-named things
   // in different translation units are not redeclarations (but may still have
-  // compatible types).
-  if (!Reader.getContext().getLangOpts().CPlusPlus)
+  // compatible types). However, clang is able to merge tag types with ODR-like
+  // semantics if it meets some requirements.
+  if (!Reader.getContext().getLangOpts().CPlusPlus &&
+      !allowODRLikeMergeInC(dyn_cast<NamedDecl>(static_cast<T*>(D))))
     return;
 
   if (FindExistingResult ExistingRes = findExisting(static_cast<T*>(D)))
Index: test/Modules/Inputs/non-ambiguous-enum/A.framework/Headers/a.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/non-ambiguous-enum/A.framework/Headers/a.h
@@ -0,0 +1 @@
+#include <A/a0.h>
Index: test/Modules/Inputs/non-ambiguous-enum/A.framework/Headers/a0.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/non-ambiguous-enum/A.framework/Headers/a0.h
@@ -0,0 +1 @@
+#include <B/b.h>
Index: test/Modules/Inputs/non-ambiguous-enum/A.framework/Modules/module.modulemap
===================================================================
--- /dev/null
+++ test/Modules/Inputs/non-ambiguous-enum/A.framework/Modules/module.modulemap
@@ -0,0 +1,6 @@
+
+framework module A {
+  header "a.h"
+  //module * { export * }
+  export *
+}
Index: test/Modules/Inputs/non-ambiguous-enum/B.framework/Headers/b.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/non-ambiguous-enum/B.framework/Headers/b.h
@@ -0,0 +1,6 @@
+typedef long NSInteger;
+typedef enum __attribute__((flag_enum,enum_extensibility(open))) MyObjCEnum : NSInteger MyObjCEnum;
+
+enum MyObjCEnum : NSInteger {
+    MyEnumCst = 1,
+} __attribute__((availability(ios,introduced=11.0))) __attribute__((availability(tvos,unavailable))) ;
Index: test/Modules/non-ambiguous-enum.m
===================================================================
--- /dev/null
+++ test/Modules/non-ambiguous-enum.m
@@ -0,0 +1,10 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -F%S/Inputs/non-ambiguous-enum -fsyntax-only %s -verify
+#import <B/B.h>
+#import <A/A.h>
+
+// expected-no-diagnostics
+
+int foo() {
+  return MyEnumCst;
+}