[Modules] Handle ObjC/C ODR-like semantics for EnumConstantDecl

Support for ObjC/C ODR-like semantics with structural equivalence
checking was added back in r306918. There enums are handled and also
checked for structural equivalence. However, at use time of
EnumConstantDecl, support was missing for preventing ambiguous
name lookup.

Add the missing bits for properly merging EnumConstantDecl.

rdar://problem/38374569

llvm-svn: 331232

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)))