Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -9166,7 +9166,7 @@
       const TemplateArgumentList &TemplateArgList,
       const TemplateArgumentListInfo &TemplateArgsInfo,
       SmallVectorImpl<TemplateArgument> &Converted,
-      SourceLocation PointOfInstantiation, void *InsertPos,
+      SourceLocation PointOfInstantiation,
       LateInstantiatedAttrVec *LateAttrs = nullptr,
       LocalInstantiationScope *StartingScope = nullptr);
   VarTemplateSpecializationDecl *CompleteVarTemplateSpecializationDecl(
Index: clang/include/clang/Sema/Template.h
===================================================================
--- clang/include/clang/Sema/Template.h
+++ clang/include/clang/Sema/Template.h
@@ -600,7 +600,7 @@
                         TagDecl *NewDecl);
 
     Decl *VisitVarTemplateSpecializationDecl(
-        VarTemplateDecl *VarTemplate, VarDecl *FromVar, void *InsertPos,
+        VarTemplateDecl *VarTemplate, VarDecl *FromVar,
         const TemplateArgumentListInfo &TemplateArgsInfo,
         ArrayRef<TemplateArgument> Converted,
         VarTemplateSpecializationDecl *PrevDecl = nullptr);
Index: clang/lib/Sema/SemaTemplate.cpp
===================================================================
--- clang/lib/Sema/SemaTemplate.cpp
+++ clang/lib/Sema/SemaTemplate.cpp
@@ -4584,7 +4584,7 @@
   // FIXME: LateAttrs et al.?
   VarTemplateSpecializationDecl *Decl = BuildVarTemplateInstantiation(
       Template, InstantiationPattern, *InstantiationArgs, TemplateArgs,
-      Converted, TemplateNameLoc, InsertPos /*, LateAttrs, StartingScope*/);
+      Converted, TemplateNameLoc /*, LateAttrs, StartingScope*/);
   if (!Decl)
     return true;
 
Index: clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -3622,11 +3622,11 @@
     return nullptr;
 
   return VisitVarTemplateSpecializationDecl(
-      InstVarTemplate, D, InsertPos, VarTemplateArgsInfo, Converted, PrevDecl);
+      InstVarTemplate, D, VarTemplateArgsInfo, Converted, PrevDecl);
 }
 
 Decl *TemplateDeclInstantiator::VisitVarTemplateSpecializationDecl(
-    VarTemplateDecl *VarTemplate, VarDecl *D, void *InsertPos,
+    VarTemplateDecl *VarTemplate, VarDecl *D,
     const TemplateArgumentListInfo &TemplateArgsInfo,
     ArrayRef<TemplateArgument> Converted,
     VarTemplateSpecializationDecl *PrevDecl) {
@@ -3649,8 +3649,11 @@
       SemaRef.Context, Owner, D->getInnerLocStart(), D->getLocation(),
       VarTemplate, DI->getType(), DI, D->getStorageClass(), Converted);
   Var->setTemplateArgsInfo(TemplateArgsInfo);
-  if (InsertPos)
+  if (!PrevDecl) {
+    void *InsertPos = nullptr;
+    VarTemplate->findSpecialization(Converted, InsertPos);
     VarTemplate->AddSpecialization(Var, InsertPos);
+  }
 
   if (SemaRef.getLangOpts().OpenCL)
     SemaRef.deduceOpenCLAddressSpace(Var);
@@ -4859,7 +4862,7 @@
     const TemplateArgumentList &TemplateArgList,
     const TemplateArgumentListInfo &TemplateArgsInfo,
     SmallVectorImpl<TemplateArgument> &Converted,
-    SourceLocation PointOfInstantiation, void *InsertPos,
+    SourceLocation PointOfInstantiation,
     LateInstantiatedAttrVec *LateAttrs,
     LocalInstantiationScope *StartingScope) {
   if (FromVar->isInvalidDecl())
@@ -4898,7 +4901,7 @@
 
   return cast_or_null<VarTemplateSpecializationDecl>(
       Instantiator.VisitVarTemplateSpecializationDecl(
-          VarTemplate, FromVar, InsertPos, TemplateArgsInfo, Converted));
+          VarTemplate, FromVar, TemplateArgsInfo, Converted));
 }
 
 /// Instantiates a variable template specialization by completing it
@@ -5321,8 +5324,8 @@
     TemplateDeclInstantiator Instantiator(*this, Var->getDeclContext(),
                                           TemplateArgs);
     Var = cast_or_null<VarDecl>(Instantiator.VisitVarTemplateSpecializationDecl(
-        VarSpec->getSpecializedTemplate(), Def, nullptr,
-        VarSpec->getTemplateArgsInfo(), VarSpec->getTemplateArgs().asArray()));
+        VarSpec->getSpecializedTemplate(), Def, VarSpec->getTemplateArgsInfo(),
+        VarSpec->getTemplateArgs().asArray(), VarSpec));
     if (Var) {
       llvm::PointerUnion<VarTemplateDecl *,
                          VarTemplatePartialSpecializationDecl *> PatternPtr =
@@ -5332,12 +5335,6 @@
         cast<VarTemplateSpecializationDecl>(Var)->setInstantiationOf(
             Partial, &VarSpec->getTemplateInstantiationArgs());
 
-      // Merge the definition with the declaration.
-      LookupResult R(*this, Var->getDeclName(), Var->getLocation(),
-                     LookupOrdinaryName, forRedeclarationInCurContext());
-      R.addDecl(OldVar);
-      MergeVarDecl(Var, R);
-
       // Attach the initializer.
       InstantiateVariableInitializer(Var, Def, TemplateArgs);
     }
Index: clang/test/SemaTemplate/instantiate-var-template.cpp
===================================================================
--- clang/test/SemaTemplate/instantiate-var-template.cpp
+++ clang/test/SemaTemplate/instantiate-var-template.cpp
@@ -40,3 +40,10 @@
   template<typename... T> A<T...> models;
   template<> struct B models<>; // expected-error {{incomplete type 'struct B'}} expected-note {{forward declaration}}
 }
+
+namespace InvalidInsertPos {
+  template<typename T, int N> T v;
+  template<int N> decltype(v<int, N-1>) v<int, N>;
+  template<> int v<int, 0>;
+  int k = v<int, 500>;
+}