[ASTImporter] Fix import of class templates partial specialization

[martong]

Summary

Currently there are several issues with the import of class template
specializations. (1) Different TUs may have class template specializations
with the same template arguments, but with different set of instantiated
MethodDecls and FieldDecls. In this patch we provide a fix to merge these
methods and fields. (2) Currently, we search the partial template
specializations in the set of simple specializations and we add partial
specializations as simple specializations. This is bad, this patch fixes it.

[a_sidorin]

Hi Gabor,

While importing methods looks harmless, importing fields can be a breaking change. Do you have any test results on this?

lib/AST/ASTImporter.cpp

2880

Honestly speaking, I wonder about this behaviour because it doesn't look similar to instantiation of only methods that are used. Is it a common rule?

4560

Importing additional fields can change the layout of the specialization. For CSA, this usually results in strange assertions hard to debug. Could you please share the results of testing of this change?
This change also doesn't seem to have related tests in this patch.

4561

The result of import is unchecked here and below. Is it intentional?

unittests/AST/ASTImporterTest.cpp

2730

3-space indentation.

2771

Broken indent.

[martong]

lib/AST/ASTImporter.cpp

2880

Yes, this is a common rule. The instantiation of an initializer is similar to the instantiation of default arguments in a sense that both are instantated only if they are being used.

To be more precise, quoting from Vandevoorde - C++ Templates The Complete Guide / 14.2.2 Instantiated Components:

... Default function call arguments are considered separately when instantiating
templates. Specifically, they are not instantiated unless there is a call to that
function (or member function) that actually makes use of the default argument.
If, on the other hand, the function is called with explicit arguments that override
the default, then the default arguments are not instantiated.
Similarly, exception specifications and default member initializers are not
instantiated unless they are needed.

4560

TLDR; We will not create additional fields.

By the time when we import the field, we already know that the existing specialization is structurally equivalent with the new one.
Since a ClassTemplateSpecializationDecl is the descendant of RecordDecl, the structural equivalence check ensures that they have the exact same fields.
When we import the field of the new spec and if there is an existing FieldDecl in the "To" context, then no new FieldDecl will be created (this is handled in VisitFieldDecl by first doing a lookup of existing field with the same name and type).
This patch extends VisitFieldDecl in a way that we add new initializer expressions to the existing FieldDecl, if it didn't have and in the "From" context it has.

For the record, I added a new test to make sure that a new FieldDecl will not be created during the merge.

4561

Yes, that is intentional. We plan to refactor all ASTImporter functions to provide a proper error handling mechanism, and in that change we would like to enforce the check of all import functions.
Unfortunately, currently we already have many places where we do not check the return value of import.

[martong]

lib/AST/ASTImporter.cpp

4561

Actually, having proper error handling is just one thing, the other more important reason why we don't check the result of the import here, because we don't want to stop merging other fields/functions if one merge failed. If one merge failed, other merge operations still could be successful.

[martong]

• Add new test
• Fix indentation

[martong]

lib/AST/ASTImporter.cpp

4560

This is the new test: ODRViolationOfClassTemplateSpecializationsShouldBeReported. It checks that it is not possible to add new fields to a specialization, rather an ODR violation is diagnosed.

[a_sidorin]

lib/AST/ASTImporter.cpp

4560

Thank you for the explanation. However, I find the comment very misleading. It tells:

// Check and merge those fields which have been instantiated
// in the "From" context, but not in the "To" context.

Would it be correct to change it to "Import field initializers that are still not instantiated", or do I still misunderstand something?

[martong]

lib/AST/ASTImporter.cpp

4560

Yes, I agree that this comment is not precise and could be misleading, thank you for pointing this out. I changed this comment and others too.

[martong]

• Change comments

[martong]

• Add one more TODO to import instantiated exception specifications

[a_sidorin]

Thank you!

include/clang/ASTMatchers/ASTMatchers.h

 ///   A<int> a;
 /// \endcode
 /// classTemplateSpecializationDecl()
 ///   matches the specializations \c A<int> and \c A<double>
 extern const internal::VariadicDynCastAllOfMatcher<
     Decl, ClassTemplateSpecializationDecl>
     classTemplateSpecializationDecl;
 
+/// Matches C++ class template partial specializations.
+///
+/// Given
+/// \code
+///   template<class T1, class T2, int I>
+///   class A {};
+///
+///   template<class T, int I>
+///   class A<T, T*, I> {};
+///
+///   template<>
+///   class A<int, int, 1> {};
+/// \endcode
+/// classTemplatePartialSpecializationDecl()
+///   matches the specialization \c A<T,T*,I> but not \c A<int,int,1>
+extern const internal::VariadicDynCastAllOfMatcher<
+    Decl, ClassTemplatePartialSpecializationDecl>
+    classTemplatePartialSpecializationDecl;
+
 /// Matches declarator declarations (field, variable, function
 /// and non-type template parameter declarations).
 ///
 /// Given
 /// \code
 ///   class X { int y; };
 /// \endcode
 /// declaratorDecl()

lib/AST/ASTImporter.cpp

if (auto *FoundField = dyn_cast<FieldDecl>(FoundDecl)) {
       if (!Name &&
           ASTImporter::getFieldIndex(D) !=
           ASTImporter::getFieldIndex(FoundField))
         continue;
 
       if (Importer.IsStructurallyEquivalent(D->getType(),
                                             FoundField->getType())) {
         Importer.MapImported(D, FoundField);
+        // In case of a FieldDecl of a ClassTemplateSpecializationDecl, the
+        // initializer of a FieldDecl might not had been instantiated in the
+        // "To" context.  However, the "From" context might instantiated that,
+        // thus we have to merge that.
+        if (Expr *FromInitializer = D->getInClassInitializer()) {
+          // We don't have yet the initializer set.
+          if (FoundField->hasInClassInitializer() &&
+              !FoundField->getInClassInitializer()) {
+            Expr *ToInitializer = Importer.Import(FromInitializer);
+            if (!ToInitializer)
+              // We can't return a nullptr here,
+              // since we already mapped D as imported.
+              return FoundField;
+            FoundField->setInClassInitializer(ToInitializer);
+          }
+        }
         return FoundField;
       }
 
       Importer.ToDiag(Loc, diag::err_odr_field_type_inconsistent)
         << Name << D->getType() << FoundField->getType();
       Importer.ToDiag(FoundField->getLocation(), diag::note_odr_value_here)
         << FoundField->getType();
       return nullptr;
Decl *ASTNodeImporter::VisitClassTemplateSpecializationDecl(
   SmallVector<TemplateArgument, 2> TemplateArgs;
   if (ImportTemplateArguments(D->getTemplateArgs().data(),
                               D->getTemplateArgs().size(),
                               TemplateArgs))
     return nullptr;
 
   // Try to find an existing specialization with these template arguments.
   void *InsertPos = nullptr;
-  ClassTemplateSpecializationDecl *D2
-    = ClassTemplate->findSpecialization(TemplateArgs, InsertPos);
-  if (D2) {
-    // We already have a class template specialization with these template
-    // arguments.
-
-    // FIXME: Check for specialization vs. instantiation errors.
-
-    if (RecordDecl *FoundDef = D2->getDefinition()) {
-      if (!D->isCompleteDefinition() || IsStructuralMatch(D, FoundDef)) {
-        // The record types structurally match, or the "from" translation
-        // unit only had a forward declaration anyway; call it the same
-        // function.
-        return Importer.MapImported(D, FoundDef);
-      }
+  ClassTemplateSpecializationDecl *D2 = nullptr;
+  ClassTemplatePartialSpecializationDecl *PartialSpec =
+            dyn_cast<ClassTemplatePartialSpecializationDecl>(D);
+  if (PartialSpec)
+    D2 = ClassTemplate->findPartialSpecialization(TemplateArgs, InsertPos);
+  else
+    D2 = ClassTemplate->findSpecialization(TemplateArgs, InsertPos);
+  ClassTemplateSpecializationDecl * const PrevDecl = D2;
+  RecordDecl *FoundDef = D2 ? D2->getDefinition() : nullptr;
+  if (FoundDef) {
+    if (!D->isCompleteDefinition()) {
+      // The "From" translation unit only had a forward declaration; call it
+      // the same declaration.
+      // TODO Handle the redecl chain properly!
+
+    if (IsStructuralMatch(D, FoundDef)) {
+
+      Importer.MapImported(D, FoundDef);
+
+      // Check and merge those fields which have been instantiated
+      // in the "From" context, but not in the "To" context.
+      for (auto *FromField : D->fields())
+        Importer.Import(FromField);
+
+      // Check and merge those methods which have been instantiated in the
+      // "From" context, but not in the "To" context.
+      for (CXXMethodDecl *FromM : D->methods())
+        Importer.Import(FromM);
+
+      // TODO  Check and merge instantiated default arguments.
+      //
+      // Generally, ASTCommon.h/DeclUpdateKind enum gives a very good hint what
+      // else could be fused during an AST merge.
+
+      return FoundDef;
     }
-  } else {
-    // Create a new specialization.
-    if (auto *PartialSpec =
-            dyn_cast<ClassTemplatePartialSpecializationDecl>(D)) {
-      // Import TemplateArgumentListInfo
+  } else { // We either couldn't find any previous specialization in the "To"
+           // context,  or we found one but without definition.  Let's create a
+           // new specialization and register that at the class template.
+    if (PartialSpec) {
+      // Import TemplateArgumentListInfo.
       TemplateArgumentListInfo ToTAInfo;
       const auto &ASTTemplateArgs = *PartialSpec->getTemplateArgsAsWritten();
       if (ImportTemplateArgumentListInfo(ASTTemplateArgs, ToTAInfo))
         return nullptr;
 
       QualType CanonInjType = Importer.Import(
             PartialSpec->getInjectedSpecializationType());
       if (CanonInjType.isNull())
         return nullptr;
       CanonInjType = CanonInjType.getCanonicalType();
 
       TemplateParameterList *ToTPList = ImportTemplateParameterList(
             PartialSpec->getTemplateParameters());
       if (!ToTPList && PartialSpec->getTemplateParameters())
         return nullptr;
 
       if (GetImportedOrCreateDecl<ClassTemplatePartialSpecializationDecl>(
               D2, D, Importer.getToContext(), D->getTagKind(), DC, StartLoc,
               IdLoc, ToTPList, ClassTemplate,
               llvm::makeArrayRef(TemplateArgs.data(), TemplateArgs.size()),
-              ToTAInfo, CanonInjType, nullptr))
+              ToTAInfo, CanonInjType,
+              cast_or_null<ClassTemplatePartialSpecializationDecl>(PrevDecl)))
         return D2;
 
-    } else {
+      // Update InsertPos, because preceding import calls may have invalidated
+      // it by adding new specializations.
+      if (!ClassTemplate->findPartialSpecialization(TemplateArgs, InsertPos))
+        // Add this partial specialization to the class template.
+        ClassTemplate->AddPartialSpecialization(
+            cast<ClassTemplatePartialSpecializationDecl>(D2), InsertPos);
+
+    } else { // Not a partial specialization.
       if (GetImportedOrCreateDecl(
               D2, D, Importer.getToContext(), D->getTagKind(), DC, StartLoc,
-              IdLoc, ClassTemplate, TemplateArgs, /*PrevDecl=*/nullptr))
+              IdLoc, ClassTemplate, TemplateArgs, PrevDecl))
         return D2;
-    }
-
-    D2->setSpecializationKind(D->getSpecializationKind());
 
+      // Update InsertPos, because preceding import calls may have invalidated
+      // it by adding new specializations.
+      if (!ClassTemplate->findSpecialization(TemplateArgs, InsertPos))
         // Add this specialization to the class template.
         ClassTemplate->AddSpecialization(D2, InsertPos);
+    }
+
+    D2->setSpecializationKind(D->getSpecializationKind());
 
     // Import the qualifier, if any.
     D2->setQualifierInfo(Importer.Import(D->getQualifierLoc()));
 
     if (auto *TSI = D->getTypeAsWritten()) {
       TypeSourceInfo *TInfo = Importer.Import(TSI);
       if (!TInfo)
         return nullptr;

lib/ASTMatchers/ASTMatchersInternal.cpp

const internal::VariadicDynCastAllOfMatcher<Decl, NamespaceAliasDecl>
     namespaceAliasDecl;
 const internal::VariadicDynCastAllOfMatcher<Decl, RecordDecl> recordDecl;
 const internal::VariadicDynCastAllOfMatcher<Decl, CXXRecordDecl> cxxRecordDecl;
 const internal::VariadicDynCastAllOfMatcher<Decl, ClassTemplateDecl>
     classTemplateDecl;
 const internal::VariadicDynCastAllOfMatcher<Decl,
                                             ClassTemplateSpecializationDecl>
     classTemplateSpecializationDecl;
+const internal::VariadicDynCastAllOfMatcher<
+    Decl, ClassTemplatePartialSpecializationDecl>
+    classTemplatePartialSpecializationDecl;
 const internal::VariadicDynCastAllOfMatcher<Decl, DeclaratorDecl>
     declaratorDecl;
 const internal::VariadicDynCastAllOfMatcher<Decl, ParmVarDecl> parmVarDecl;
 const internal::VariadicDynCastAllOfMatcher<Decl, AccessSpecDecl>
     accessSpecDecl;
 const internal::VariadicAllOfMatcher<CXXCtorInitializer> cxxCtorInitializer;
 const internal::VariadicAllOfMatcher<TemplateArgument> templateArgument;
 const internal::VariadicAllOfMatcher<TemplateName> templateName;

unittests/AST/ASTImporterTest.cpp

for (auto *FromField : FromLambda->fields()) {
     unsigned ToIndex = ASTImporter::getFieldIndex(ToField);
     EXPECT_EQ(ToIndex, FromIndex + 1);
     ++FromIndex;
   }
 
   EXPECT_EQ(FromIndex, 3u);
 }
 
+TEST_P(ASTImporterTestBase, MergeFieldDeclsOfClassTemplateSpecialization) {
+  std::string ClassTemplate =
+      R"(
+      template <typename T>
+      struct X {
+          int a{0}; // FieldDecl with InitListExpr
+          X(char) : a(3) {}     // (1)
+          X(int) {}             // (2)
+      };
+      )";
+  Decl *ToTU = getToTuDecl(ClassTemplate +
+      R"(
+      void foo() {
+          // ClassTemplateSpec with ctor (1): FieldDecl without InitlistExpr
+          X<char> xc('c');
+      }
+      )", Lang_CXX11);
+  auto *ToSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      ToTU, classTemplateSpecializationDecl(hasName("X")));
+  // FieldDecl without InitlistExpr:
+  auto *ToField = *ToSpec->field_begin();
+  ASSERT_TRUE(ToField);
+  ASSERT_FALSE(ToField->getInClassInitializer());
+  Decl *FromTU = getTuDecl(ClassTemplate +
+      R"(
+      void bar() {
+          // ClassTemplateSpec with ctor (2): FieldDecl WITH InitlistExpr
+          X<char> xc(1);
+      }
+      )", Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl(hasName("X")));
+  // FieldDecl with InitlistExpr:
+  auto *FromField = *FromSpec->field_begin();
+  ASSERT_TRUE(FromField);
+  ASSERT_TRUE(FromField->getInClassInitializer());
+
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  ASSERT_TRUE(ImportedSpec);
+  EXPECT_EQ(ImportedSpec, ToSpec);
+  // After the import, the FieldDecl has to be merged, thus it should have the
+  // InitListExpr.
+  EXPECT_TRUE(ToField->getInClassInitializer());
+}
+
+TEST_P(ASTImporterTestBase, MergeFunctionOfClassTemplateSpecialization) {
+  std::string ClassTemplate =
+      R"(
+      template <typename T>
+      struct X {
+        void f() {}
+        void g() {}
+      };
+      )";
+  Decl *ToTU = getToTuDecl(ClassTemplate +
+      R"(
+      void foo() {
+          X<char> x;
+          x.f();
+      }
+      )", Lang_CXX11);
+  Decl *FromTU = getTuDecl(ClassTemplate +
+      R"(
+      void bar() {
+          X<char> x;
+          x.g();
+      }
+      )", Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl(hasName("X")));
+  auto FunPattern = functionDecl(hasName("g"),
+                         hasParent(classTemplateSpecializationDecl()));
+  auto *FromFun =
+      FirstDeclMatcher<FunctionDecl>().match(FromTU, FunPattern);
+  auto *ToFun =
+      FirstDeclMatcher<FunctionDecl>().match(ToTU, FunPattern);
+  ASSERT_TRUE(FromFun->hasBody());
+  ASSERT_FALSE(ToFun->hasBody());
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  ASSERT_TRUE(ImportedSpec);
+  auto *ToSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      ToTU, classTemplateSpecializationDecl(hasName("X")));
+  EXPECT_EQ(ImportedSpec, ToSpec);
+  EXPECT_TRUE(ToFun->hasBody());
+}
+
+TEST_P(ASTImporterTestBase,
+       ODRViolationOfClassTemplateSpecializationsShouldBeReported) {
+  std::string ClassTemplate =
+      R"(
+      template <typename T>
+      struct X {};
+      )";
+  Decl *ToTU = getToTuDecl(ClassTemplate +
+                               R"(
+      template <>
+      struct X<char> {
+          int a;
+      };
+      void foo() {
+          X<char> x;
+      }
+      )",
+                           Lang_CXX11);
+  Decl *FromTU = getTuDecl(ClassTemplate +
+                               R"(
+      template <>
+      struct X<char> {
+          int b;
+      };
+      void foo() {
+          X<char> x;
+      }
+      )",
+                           Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl(hasName("X")));
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+
+  // We expect one (ODR) warning during the import.
+  EXPECT_EQ(1u, ToTU->getASTContext().getDiagnostics().getNumWarnings());
+
+  // The second specialization is different from the first, thus it violates
+  // ODR, consequently we expect to keep the first specialization only, which is
+  // already in the "To" context.
+  EXPECT_TRUE(ImportedSpec);
+  auto *ToSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      ToTU, classTemplateSpecializationDecl(hasName("X")));
+  EXPECT_EQ(ImportedSpec, ToSpec);
+  EXPECT_EQ(1u, DeclCounter<ClassTemplateSpecializationDecl>().match(
+                    ToTU, classTemplateSpecializationDecl()));
+}
+
+TEST_P(ASTImporterTestBase, MergeCtorOfClassTemplateSpecialization) {
+  std::string ClassTemplate =
+      R"(
+      template <typename T>
+      struct X {
+          X(char) {}
+          X(int) {}
+      };
+      )";
+  Decl *ToTU = getToTuDecl(ClassTemplate +
+      R"(
+      void foo() {
+          X<char> x('c');
+      }
+      )", Lang_CXX11);
+  Decl *FromTU = getTuDecl(ClassTemplate +
+      R"(
+      void bar() {
+          X<char> x(1);
+      }
+      )", Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl(hasName("X")));
+  // Match the void(int) ctor.
+  auto CtorPattern =
+      cxxConstructorDecl(hasParameter(0, varDecl(hasType(asString("int")))),
+                         hasParent(classTemplateSpecializationDecl()));
+  auto *FromCtor =
+      FirstDeclMatcher<CXXConstructorDecl>().match(FromTU, CtorPattern);
+  auto *ToCtor =
+      FirstDeclMatcher<CXXConstructorDecl>().match(ToTU, CtorPattern);
+  ASSERT_TRUE(FromCtor->hasBody());
+  ASSERT_FALSE(ToCtor->hasBody());
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  ASSERT_TRUE(ImportedSpec);
+  auto *ToSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      ToTU, classTemplateSpecializationDecl(hasName("X")));
+  EXPECT_EQ(ImportedSpec, ToSpec);
+  EXPECT_TRUE(ToCtor->hasBody());
+}
+
+TEST_P(ASTImporterTestBase,
+       ClassTemplatePartialSpecializationsShouldNotBeDuplicated) {
+  auto Code =
+      R"(
+    // primary template
+    template<class T1, class T2, int I>
+    class A {};
+
+    // partial specialization
+    template<class T, int I>
+    class A<T, T*, I> {};
+    )";
+  Decl *ToTU = getToTuDecl(Code, Lang_CXX11);
+  Decl *FromTU = getTuDecl(Code, Lang_CXX11);
+  auto *FromSpec =
+      FirstDeclMatcher<ClassTemplatePartialSpecializationDecl>().match(
+          FromTU, classTemplatePartialSpecializationDecl());
+  auto *ToSpec =
+      FirstDeclMatcher<ClassTemplatePartialSpecializationDecl>().match(
+          ToTU, classTemplatePartialSpecializationDecl());
+
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  EXPECT_EQ(ImportedSpec, ToSpec);
+  EXPECT_EQ(1u, DeclCounter<ClassTemplatePartialSpecializationDecl>().match(
+                    ToTU, classTemplatePartialSpecializationDecl()));
+}
+
+TEST_P(ASTImporterTestBase, ClassTemplateSpecializationsShouldNotBeDuplicated) {
+  auto Code =
+      R"(
+    // primary template
+    template<class T1, class T2, int I>
+    class A {};
+
+    // full specialization
+    template<>
+    class A<int, int, 1> {};
+    )";
+  Decl *ToTU = getToTuDecl(Code, Lang_CXX11);
+  Decl *FromTU = getTuDecl(Code, Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl());
+  auto *ToSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      ToTU, classTemplateSpecializationDecl());
+
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  EXPECT_EQ(ImportedSpec, ToSpec);
+  EXPECT_EQ(1u, DeclCounter<ClassTemplateSpecializationDecl>().match(
+                   ToTU, classTemplateSpecializationDecl()));
+}
+
+TEST_P(ASTImporterTestBase, ClassTemplateFullAndPartialSpecsShouldNotBeMixed) {
+  std::string PrimaryTemplate =
+      R"(
+    template<class T1, class T2, int I>
+    class A {};
+    )";
+  auto PartialSpec =
+      R"(
+    template<class T, int I>
+    class A<T, T*, I> {};
+    )";
+  auto FullSpec =
+      R"(
+    template<>
+    class A<int, int, 1> {};
+    )";
+  Decl *ToTU = getToTuDecl(PrimaryTemplate + FullSpec, Lang_CXX11);
+  Decl *FromTU = getTuDecl(PrimaryTemplate + PartialSpec, Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl());
+
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  EXPECT_TRUE(ImportedSpec);
+  // Check the number of partial specializations.
+  EXPECT_EQ(1u, DeclCounter<ClassTemplatePartialSpecializationDecl>().match(
+                    ToTU, classTemplatePartialSpecializationDecl()));
+  // Check the number of full specializations.
+  EXPECT_EQ(1u, DeclCounter<ClassTemplateSpecializationDecl>().match(
+                    ToTU, classTemplateSpecializationDecl(
+                              unless(classTemplatePartialSpecializationDecl()))));
+}
+
 struct DeclContextTest : ASTImporterTestBase {};
 
 TEST_P(DeclContextTest, removeDeclOfClassTemplateSpecialization) {
   Decl *TU = getTuDecl(
       R"(
       namespace NS {
 
       template <typename T>