[SemaTemplate] Stop passing insertion position around during VarTemplate instantiation

They can get stale at use time because of updates from other recursive
specializations. Instead, rely on the existence of previous declarations to add
the specialization.

Differential Revision: https://reviews.llvm.org/D87853

(cherry picked from commit cffb0dd)

This is a re-commit of 8ac7095 with
some modifications to avoid changing the clang API.

Author: bcardosolopes

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -3629,8 +3629,11 @@ Decl *TemplateDeclInstantiator::VisitVarTemplateSpecializationDecl(
       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);
@@ -5311,7 +5314,7 @@ void Sema::InstantiateVariableDefinition(SourceLocation PointOfInstantiation,
                                           TemplateArgs);
     Var = cast_or_null<VarDecl>(Instantiator.VisitVarTemplateSpecializationDecl(
         VarSpec->getSpecializedTemplate(), Def, nullptr,
-        VarSpec->getTemplateArgsInfo(), VarSpec->getTemplateArgs().asArray()));
+        VarSpec->getTemplateArgsInfo(), VarSpec->getTemplateArgs().asArray(), VarSpec));
     if (Var) {
       llvm::PointerUnion<VarTemplateDecl *,
                          VarTemplatePartialSpecializationDecl *> PatternPtr =
@@ -5321,12 +5324,6 @@ void Sema::InstantiateVariableDefinition(SourceLocation PointOfInstantiation,
         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);
     }

clang/test/SemaTemplate/instantiate-var-template.cpp

@@ -40,3 +40,10 @@ namespace PR24483 {
   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>;
+}