5 files changed, 19 insertions, 5 deletions
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 41c4fd0e6c0..29846b61010 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -2029,9 +2029,12 @@ VarDecl::isThisDeclarationADefinition(ASTContext &C) const {
   // A variable template specialization (other than a static data member
   // template or an explicit specialization) is a declaration until we
   // instantiate its initializer.
-  if (isa<VarTemplateSpecializationDecl>(this) &&
-      getTemplateSpecializationKind() != TSK_ExplicitSpecialization)
-    return DeclarationOnly;
+  if (auto *VTSD = dyn_cast<VarTemplateSpecializationDecl>(this)) {
+    if (VTSD->getTemplateSpecializationKind() != TSK_ExplicitSpecialization &&
+        !isa<VarTemplatePartialSpecializationDecl>(VTSD) &&
+        !VTSD->IsCompleteDefinition)
+      return DeclarationOnly;
+  }
 
   if (hasExternalStorage())
     return DeclarationOnly;
diff --git a/clang/lib/AST/DeclTemplate.cpp b/clang/lib/AST/DeclTemplate.cpp
index f5a2ab07bf7..54e8dd86624 100644
--- a/clang/lib/AST/DeclTemplate.cpp
+++ b/clang/lib/AST/DeclTemplate.cpp
@@ -1043,13 +1043,13 @@ VarTemplateSpecializationDecl::VarTemplateSpecializationDecl(
               SpecializedTemplate->getIdentifier(), T, TInfo, S),
       SpecializedTemplate(SpecializedTemplate),
       TemplateArgs(TemplateArgumentList::CreateCopy(Context, Args)),
-      SpecializationKind(TSK_Undeclared) {}
+      SpecializationKind(TSK_Undeclared), IsCompleteDefinition(false) {}
 
 VarTemplateSpecializationDecl::VarTemplateSpecializationDecl(Kind DK,
                                                              ASTContext &C)
     : VarDecl(DK, C, nullptr, SourceLocation(), SourceLocation(), nullptr,
               QualType(), nullptr, SC_None),
-      SpecializationKind(TSK_Undeclared) {}
+      SpecializationKind(TSK_Undeclared), IsCompleteDefinition(false) {}
 
 VarTemplateSpecializationDecl *VarTemplateSpecializationDecl::Create(
     ASTContext &Context, DeclContext *DC, SourceLocation StartLoc,
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 5b528fa9c2e..eacb325642e 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -4021,6 +4021,8 @@ VarTemplateSpecializationDecl *Sema::BuildVarTemplateInstantiation(
 VarTemplateSpecializationDecl *Sema::CompleteVarTemplateSpecializationDecl(
     VarTemplateSpecializationDecl *VarSpec, VarDecl *PatternDecl,
     const MultiLevelTemplateArgumentList &TemplateArgs) {
+  assert(PatternDecl->isThisDeclarationADefinition() &&
+         "don't have a definition to instantiate from");
 
   // Do substitution on the type of the declaration
   TypeSourceInfo *DI =
@@ -4032,6 +4034,9 @@ VarTemplateSpecializationDecl *Sema::CompleteVarTemplateSpecializationDecl(
   // Update the type of this variable template specialization.
   VarSpec->setType(DI->getType());
 
+  // Convert the declaration into a definition now.
+  VarSpec->setCompleteDefinition();
+
   // Instantiate the initializer.
   InstantiateVariableInitializer(VarSpec, PatternDecl, TemplateArgs);
 
@@ -4225,6 +4230,10 @@ void Sema::InstantiateVariableDefinition(SourceLocation PointOfInstantiation,
   if (Var->isInvalidDecl())
     return;
 
+  // FIXME: We're missing ASTMutationListener notifications for all of the work
+  // done here. (Some of our callers notify the listeners for the static data
+  // member case, but not in general.)
+
   VarTemplateSpecializationDecl *VarSpec =
       dyn_cast<VarTemplateSpecializationDecl>(Var);
   VarDecl *PatternDecl = nullptr, *Def = nullptr;
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 126738b6173..eb7c1777ac6 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -2198,6 +2198,7 @@ ASTDeclReader::VisitVarTemplateSpecializationDeclImpl(
   D->TemplateArgs = TemplateArgumentList::CreateCopy(C, TemplArgs);
   D->PointOfInstantiation = ReadSourceLocation();
   D->SpecializationKind = (TemplateSpecializationKind)Record.readInt();
+  D->IsCompleteDefinition = Record.readInt();
 
   bool writtenAsCanonicalDecl = Record.readInt();
   if (writtenAsCanonicalDecl) {
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index fcf78e850b2..041ccd47d03 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -1494,6 +1494,7 @@ void ASTDeclWriter::VisitVarTemplateSpecializationDecl(
   Record.AddTemplateArgumentList(&D->getTemplateArgs());
   Record.AddSourceLocation(D->getPointOfInstantiation());
   Record.push_back(D->getSpecializationKind());
+  Record.push_back(D->IsCompleteDefinition);
   Record.push_back(D->isCanonicalDecl());
 
   if (D->isCanonicalDecl()) {