We were accidentally giving value witnesses default visibility,

which doesn't play well with others.

Swift SVN r1996

Author: rjmccall

lib/IRGen/GenDecl.cpp

@@ -120,10 +120,18 @@ LinkInfo LinkInfo::get(IRGenModule &IGM, const LinkEntity &entity) {
 
   llvm::raw_svector_ostream nameStream(result.Name);
   entity.mangle(nameStream);
-  
-  // TODO, obviously.
-  result.Linkage = llvm::GlobalValue::ExternalLinkage;
-  result.Visibility = llvm::GlobalValue::DefaultVisibility;
+
+  // The linkage for a value witness is linkonce_odr.
+  if (entity.isValueWitness()) {
+    result.Linkage = llvm::GlobalValue::LinkOnceODRLinkage;
+    result.Visibility = llvm::GlobalValue::HiddenVisibility;
+
+  // Give everything else external linkage.
+  } else {
+    result.Linkage = llvm::GlobalValue::ExternalLinkage;
+    result.Visibility = llvm::GlobalValue::DefaultVisibility;
+  }
+
   return result;
 }
 

lib/IRGen/Linking.h

@@ -153,6 +153,7 @@ class LinkEntity {
     return LINKENTITY_GET_FIELD(Data, UncurryLevel);
   }
 
+  bool isValueWitness() const { return getKind() == Kind::ValueWitness; }
   Type getType() const {
     assert(isTypeKind(getKind()));
     return reinterpret_cast<TypeBase*>(Pointer);