Initial support for variable templates

robertoraggi · robertoraggi · commit 68bb271b2d4a · 2025-08-28T18:35:53.000+02:00
Signed-off-by: Roberto Raggi &lt;roberto.raggi@gmail.com&gt;
diff --git a/src/parser/cxx/ast_rewriter.cc b/src/parser/cxx/ast_rewriter.cc
@@ -201,6 +201,68 @@ auto ASTRewriter::instantiateTypeAliasTemplate(
   return instance->symbol;
 }
 
+auto ASTRewriter::instantiateVariableTemplate(
+    TranslationUnit* unit, List<TemplateArgumentAST*>* templateArgumentList,
+    VariableSymbol* variableSymbol) -> VariableSymbol* {
+  auto templateDecl = variableSymbol->templateDeclaration();
+
+  if (!templateDecl) {
+    unit->error(variableSymbol->location(), "not a template");
+    return nullptr;
+  }
+
+  auto variableDeclaration =
+      ast_cast<SimpleDeclarationAST>(templateDecl->declaration);
+
+  if (!variableDeclaration) return nullptr;
+
+  auto templateArguments =
+      make_substitution(unit, templateDecl, templateArgumentList);
+
+  auto is_primary_template = [&]() -> bool {
+    int expected = 0;
+    for (const auto& arg : templateArguments) {
+      if (!std::holds_alternative<Symbol*>(arg)) return false;
+
+      auto ty = type_cast<TypeParameterType>(std::get<Symbol*>(arg)->type());
+      if (!ty) return false;
+
+      if (ty->index() != expected) return false;
+      ++expected;
+    }
+    return true;
+  };
+
+  if (is_primary_template()) {
+    // if this is a primary template, we can just return the class symbol
+    return variableSymbol;
+  }
+
+  auto subst = variableSymbol->findSpecialization(templateArguments);
+  if (subst) {
+    return subst;
+  }
+
+  auto parentScope = variableSymbol->parent();
+  while (parentScope->isTemplateParameters()) {
+    parentScope = parentScope->parent();
+  }
+
+  auto rewriter = ASTRewriter{unit, parentScope, templateArguments};
+
+  rewriter.binder().setInstantiatingSymbol(variableSymbol);
+
+  auto instance =
+      ast_cast<SimpleDeclarationAST>(rewriter.declaration(variableDeclaration));
+
+  if (!instance) return nullptr;
+
+  auto instantiatedSymbol = instance->initDeclaratorList->value->symbol;
+  auto instantiatedVariable = symbol_cast<VariableSymbol>(instantiatedSymbol);
+
+  return instantiatedVariable;
+}
+
 auto ASTRewriter::make_substitution(
     TranslationUnit* unit, TemplateDeclarationAST* templateDecl,
     List<TemplateArgumentAST*>* templateArgumentList)
diff --git a/src/parser/cxx/ast_rewriter.h b/src/parser/cxx/ast_rewriter.h
@@ -42,6 +42,10 @@ class ASTRewriter {
       TranslationUnit* unit, List<TemplateArgumentAST*>* templateArgumentList,
       TypeAliasSymbol* typeAliasSymbol) -> TypeAliasSymbol*;
 
+  [[nodiscard]] static auto instantiateVariableTemplate(
+      TranslationUnit* unit, List<TemplateArgumentAST*>* templateArgumentList,
+      VariableSymbol* variableSymbol) -> VariableSymbol*;
+
   explicit ASTRewriter(TranslationUnit* unit, ScopeSymbol* scope,
                        const std::vector<TemplateArgument>& templateArguments);
   ~ASTRewriter();
diff --git a/src/parser/cxx/ast_rewriter_declarators.cc b/src/parser/cxx/ast_rewriter_declarators.cc
@@ -22,6 +22,7 @@
 
 // cxx
 #include <cxx/ast.h>
+#include <cxx/ast_interpreter.h>
 #include <cxx/binder.h>
 #include <cxx/decl.h>
 #include <cxx/decl_specs.h>
@@ -208,6 +209,9 @@ auto ASTRewriter::initDeclarator(InitDeclaratorAST* ast,
   auto type =
       getDeclaratorType(translationUnit(), copy->declarator, declSpecs.type());
 
+  const auto addSymbolToParentScope =
+      binder().instantiatingSymbol() != ast->symbol;
+
   // ### fix scope
   if (binder_.scope()->isClass()) {
     auto symbol = binder_.declareMemberSymbol(copy->declarator, decl);
@@ -222,16 +226,30 @@ auto ASTRewriter::initDeclarator(InitDeclaratorAST* ast,
         auto functionSymbol = binder_.declareFunction(copy->declarator, decl);
         copy->symbol = functionSymbol;
       } else {
-        auto variableSymbol = binder_.declareVariable(copy->declarator, decl);
+        auto variableSymbol = binder_.declareVariable(copy->declarator, decl,
+                                                      addSymbolToParentScope);
         // variableSymbol->setTemplateDeclaration(templateHead);
         copy->symbol = variableSymbol;
+
+        if (!addSymbolToParentScope) {
+          auto templateVariable = symbol_cast<VariableSymbol>(ast->symbol);
+          templateVariable->addSpecialization(templateArguments(),
+                                              variableSymbol);
+        }
       }
     }
   }
 
   copy->requiresClause = requiresClause(ast->requiresClause);
   copy->initializer = expression(ast->initializer);
-  // copy->symbol = ast->symbol; // TODO remove, done above
+
+  if (auto variableSymbol = symbol_cast<VariableSymbol>(copy->symbol)) {
+    if (variableSymbol->isConstexpr()) {
+      auto interp = ASTInterpreter{unit_};
+      auto constValue = interp.evaluate(copy->initializer);
+      variableSymbol->setConstValue(constValue);
+    }
+  }
 
   return copy;
 }
diff --git a/src/parser/cxx/binder.cc b/src/parser/cxx/binder.cc
@@ -818,15 +818,17 @@ auto Binder::declareField(DeclaratorAST* declarator, const Decl& decl)
   return fieldSymbol;
 }
 
-auto Binder::declareVariable(DeclaratorAST* declarator, const Decl& decl)
-    -> VariableSymbol* {
+auto Binder::declareVariable(DeclaratorAST* declarator, const Decl& decl,
+                             bool addSymbolToParentScope) -> VariableSymbol* {
   auto name = decl.getName();
   auto symbol = control()->newVariableSymbol(scope(), decl.location());
   auto type = getDeclaratorType(unit_, declarator, decl.specs.type());
   applySpecifiers(symbol, decl.specs);
   symbol->setName(name);
   symbol->setType(type);
-  declaringScope()->addSymbol(symbol);
+  if (addSymbolToParentScope) {
+    declaringScope()->addSymbol(symbol);
+  }
   return symbol;
 }
 
@@ -963,6 +965,21 @@ void Binder::bind(IdExpressionAST* ast) {
   }
 
   ast->symbol = Lookup{scope()}(ast->nestedNameSpecifier, componentName);
+
+  if (unit_->config().checkTypes) {
+    if (auto templateId = ast_cast<SimpleTemplateIdAST>(ast->unqualifiedId)) {
+      auto var = symbol_cast<VariableSymbol>(ast->symbol);
+
+      if (!var) {
+        error(templateId->firstSourceLocation(), std::format("not a template"));
+      } else {
+        auto instance = ASTRewriter::instantiateVariableTemplate(
+            unit_, templateId->templateArgumentList, var);
+
+        ast->symbol = instance;
+      }
+    }
+  }
 }
 
 auto Binder::getFunction(ScopeSymbol* scope, const Name* name, const Type* type)
diff --git a/src/parser/cxx/binder.h b/src/parser/cxx/binder.h
@@ -82,7 +82,9 @@ class Binder {
       -> FieldSymbol*;
 
   [[nodiscard]] auto declareVariable(DeclaratorAST* declarator,
-                                     const Decl& decl) -> VariableSymbol*;
+                                     const Decl& decl,
+                                     bool addSymbolToParentScope)
+      -> VariableSymbol*;
 
   [[nodiscard]] auto declareMemberSymbol(DeclaratorAST* declarator,
                                          const Decl& decl) -> Symbol*;
diff --git a/src/parser/cxx/parser.cc b/src/parser/cxx/parser.cc
@@ -3348,7 +3348,8 @@ void Parser::parse_condition(ExpressionAST*& yyast, const ExprContext& ctx) {
 
     if (!parse_declarator(declarator, decl)) return false;
 
-    auto symbol = binder_.declareVariable(declarator, decl);
+    auto symbol = binder_.declareVariable(declarator, decl,
+                                          /*addSymbolToParentScope=*/true);
 
     ExpressionAST* initializer = nullptr;
 
@@ -5573,7 +5574,8 @@ auto Parser::parse_init_declarator(InitDeclaratorAST*& yyast,
       auto functionSymbol = binder_.declareFunction(declarator, decl);
       symbol = functionSymbol;
     } else {
-      auto variableSymbol = binder_.declareVariable(declarator, decl);
+      auto variableSymbol = binder_.declareVariable(
+          declarator, decl, /*addSymbolToParentScope=*/true);
       variableSymbol->setTemplateDeclaration(templateHead);
       symbol = variableSymbol;
     }
diff --git a/src/parser/cxx/symbols.cc b/src/parser/cxx/symbols.cc
@@ -769,6 +769,28 @@ void VariableSymbol::setTemplateDeclaration(
   templateDeclaration_ = declaration;
 }
 
+auto VariableSymbol::specializations() const
+    -> std::span<const TemplateSpecialization<VariableSymbol>> {
+  if (!templateInfo_) return {};
+  return templateInfo_->specializations();
+}
+
+auto VariableSymbol::findSpecialization(
+    const std::vector<TemplateArgument>& arguments) const -> VariableSymbol* {
+  if (!templateInfo_) return {};
+  return templateInfo_->findSpecialization(arguments);
+}
+
+void VariableSymbol::addSpecialization(std::vector<TemplateArgument> arguments,
+                                       VariableSymbol* specialization) {
+  if (!templateInfo_) {
+    templateInfo_ = std::make_unique<TemplateInfo<VariableSymbol>>(this);
+  }
+  auto index = templateInfo_->specializations().size();
+  specialization->setSpecializationInfo(this, index);
+  templateInfo_->addSpecialization(std::move(arguments), specialization);
+}
+
 auto VariableSymbol::initializer() const -> ExpressionAST* {
   return initializer_;
 }
diff --git a/src/parser/cxx/symbols.h b/src/parser/cxx/symbols.h
@@ -595,6 +595,28 @@ class VariableSymbol final : public Symbol {
   [[nodiscard]] auto templateDeclaration() const -> TemplateDeclarationAST*;
   void setTemplateDeclaration(TemplateDeclarationAST* declaration);
 
+  [[nodiscard]] auto specializations() const
+      -> std::span<const TemplateSpecialization<VariableSymbol>>;
+
+  [[nodiscard]] auto findSpecialization(
+      const std::vector<TemplateArgument>& arguments) const -> VariableSymbol*;
+
+  void addSpecialization(std::vector<TemplateArgument> arguments,
+                         VariableSymbol* specialization);
+
+  void setSpecializationInfo(VariableSymbol* templateVariable,
+                             std::size_t index) {
+    templateVariable_ = templateVariable;
+    templateSepcializationIndex_ = index;
+  }
+
+  [[nodiscard]] auto templateArguments() const
+      -> std::span<const TemplateArgument> {
+    if (!templateVariable_) return {};
+    return templateVariable_->specializations()[templateSepcializationIndex_]
+        .arguments;
+  }
+
   [[nodiscard]] auto initializer() const -> ExpressionAST*;
   void setInitializer(ExpressionAST*);
 
@@ -605,6 +627,9 @@ class VariableSymbol final : public Symbol {
   TemplateDeclarationAST* templateDeclaration_ = nullptr;
   ExpressionAST* initializer_ = nullptr;
   std::optional<ConstValue> constValue_;
+  std::unique_ptr<TemplateInfo<VariableSymbol>> templateInfo_;
+  VariableSymbol* templateVariable_ = nullptr;
+  std::size_t templateSepcializationIndex_ = 0;
 
   union {
     std::uint32_t flags_{};
diff --git a/tests/unit_tests/sema/template_var_01.cc b/tests/unit_tests/sema/template_var_01.cc
@@ -0,0 +1,8 @@
+// RUN: %cxx -verify -fcheck -dump-symbols %s
+
+template <typename T>
+constexpr bool is_integral_v = __is_integral(T);
+
+static_assert(is_integral_v<int>);
+static_assert(is_integral_v<char>);
+static_assert(is_integral_v<void*> == false);
