[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

clang/include/clang/Sema/Sema.h

@@ -9171,7 +9171,7 @@ public:
       const TemplateArgumentList &TemplateArgList,
       const TemplateArgumentListInfo &TemplateArgsInfo,
       SmallVectorImpl<TemplateArgument> &Converted,
-      SourceLocation PointOfInstantiation, void *InsertPos,
+      SourceLocation PointOfInstantiation,
       LateInstantiatedAttrVec *LateAttrs = nullptr,
       LocalInstantiationScope *StartingScope = nullptr);
   VarTemplateSpecializationDecl *CompleteVarTemplateSpecializationDecl(

clang/include/clang/Sema/Template.h

@@ -600,7 +600,7 @@ enum class TemplateSubstitutionKind : char {
                         TagDecl *NewDecl);
 
     Decl *VisitVarTemplateSpecializationDecl(
-        VarTemplateDecl *VarTemplate, VarDecl *FromVar, void *InsertPos,
+        VarTemplateDecl *VarTemplate, VarDecl *FromVar,
         const TemplateArgumentListInfo &TemplateArgsInfo,
         ArrayRef<TemplateArgument> Converted,
         VarTemplateSpecializationDecl *PrevDecl = nullptr);

clang/lib/Sema/SemaTemplate.cpp

@@ -4584,7 +4584,7 @@ Sema::CheckVarTemplateId(VarTemplateDecl *Template, SourceLocation TemplateLoc,
   // FIXME: LateAttrs et al.?
   VarTemplateSpecializationDecl *Decl = BuildVarTemplateInstantiation(
       Template, InstantiationPattern, *InstantiationArgs, TemplateArgs,
-      Converted, TemplateNameLoc, InsertPos /*, LateAttrs, StartingScope*/);
+      Converted, TemplateNameLoc /*, LateAttrs, StartingScope*/);
   if (!Decl)
     return true;
 

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -3628,11 +3628,11 @@ Decl *TemplateDeclInstantiator::VisitVarTemplateSpecializationDecl(
     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) {
@@ -3655,8 +3655,11 @@ 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);
@@ -4865,7 +4868,7 @@ VarTemplateSpecializationDecl *Sema::BuildVarTemplateInstantiation(
     const TemplateArgumentList &TemplateArgList,
     const TemplateArgumentListInfo &TemplateArgsInfo,
     SmallVectorImpl<TemplateArgument> &Converted,
-    SourceLocation PointOfInstantiation, void *InsertPos,
+    SourceLocation PointOfInstantiation,
     LateInstantiatedAttrVec *LateAttrs,
     LocalInstantiationScope *StartingScope) {
   if (FromVar->isInvalidDecl())
@@ -4904,7 +4907,7 @@ VarTemplateSpecializationDecl *Sema::BuildVarTemplateInstantiation(
 
   return cast_or_null<VarTemplateSpecializationDecl>(
       Instantiator.VisitVarTemplateSpecializationDecl(
-          VarTemplate, FromVar, InsertPos, TemplateArgsInfo, Converted));
+          VarTemplate, FromVar, TemplateArgsInfo, Converted));
 }
 
 /// Instantiates a variable template specialization by completing it
@@ -5327,8 +5330,8 @@ void Sema::InstantiateVariableDefinition(SourceLocation PointOfInstantiation,
     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 =
@@ -5338,12 +5341,6 @@ 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>;
+}