1 files changed, 20 insertions, 4 deletions
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 046c699a224..5b86c0a777d 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -2547,6 +2547,20 @@ void ASTDeclReader::mergeRedeclarable(Redeclarable<T> *DBase, T *Existing,
   }
 }
 
+/// 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
@@ -2557,10 +2571,12 @@ void ASTDeclReader::mergeMergeable(Mergeable<T> *D) {
   if (!Reader.getContext().getLangOpts().Modules)
     return;
 
-  // 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)
+  // ODR-based merging is performed in C++ and in some cases (tag types) in C.
+  // Note that C identically-named things in different translation units are
+  // not redeclarations, but may still have compatible types, where ODR-like
+  // semantics may apply.
+  if (!Reader.getContext().getLangOpts().CPlusPlus &&
+      !allowODRLikeMergeInC(dyn_cast<NamedDecl>(static_cast<T*>(D))))
     return;
 
   if (FindExistingResult ExistingRes = findExisting(static_cast<T*>(D)))