Switch Member to use inheritance+cast (#713)

executable_semantics/ast/member.cpp

@@ -5,26 +5,17 @@
 #include "executable_semantics/ast/member.h"
 
 #include "executable_semantics/common/arena.h"
+#include "llvm/Support/Casting.h"
 
 namespace Carbon {
 
-auto Member::MakeFieldMember(int line_num, const BindingPattern* binding)
-    -> Member* {
-  auto m = global_arena->New<Member>();
-  m->line_num = line_num;
-  m->value = FieldMember({.binding = binding});
-  return m;
-}
-
-auto Member::GetFieldMember() const -> const FieldMember& {
-  return std::get<FieldMember>(value);
-}
+using llvm::cast;
 
 void Member::Print(llvm::raw_ostream& out) const {
-  switch (tag()) {
-    case MemberKind::FieldMember:
-      const auto& field = GetFieldMember();
-      out << "var " << field.binding << ";\n";
+  switch (Tag()) {
+    case Kind::FieldMember:
+      const auto& field = cast<FieldMember>(*this);
+      out << "var " << field.Binding() << ";\n";
       break;
   }
 }

executable_semantics/ast/member.h

@@ -14,35 +14,58 @@
 
 namespace Carbon {
 
-enum class MemberKind { FieldMember };
+// Abstract base class of all AST nodes representing patterns.
+//
+// Member and its derived classes support LLVM-style RTTI, including
+// llvm::isa, llvm::cast, and llvm::dyn_cast. To support this, every
+// class derived from Member must provide a `classof` operation, and
+// every concrete derived class must have a corresponding enumerator
+// in `Kind`; see https://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html for
+// details.
+class Member {
+ public:
+  enum class Kind { FieldMember };
 
-struct FieldMember {
-  static constexpr MemberKind Kind = MemberKind::FieldMember;
+  Member(const Member&) = delete;
+  Member& operator=(const Member&) = delete;
+
+  // Returns the enumerator corresponding to the most-derived type of this
+  // object.
+  auto Tag() const -> Kind { return tag; }
+
+  auto LineNumber() const -> int { return line_num; }
+
+  void Print(llvm::raw_ostream& out) const;
+
+ protected:
+  // Constructs a Member representing syntax at the given line number.
+  // `tag` must be the enumerator corresponding to the most-derived type being
+  // constructed.
+  Member(Kind tag, int line_num) : tag(tag), line_num(line_num) {}
+
+ private:
+  const Kind tag;
+  int line_num;
+};
+
+class FieldMember : public Member {
+ public:
+  FieldMember(int line_num, const BindingPattern* binding)
+      : Member(Kind::FieldMember, line_num), binding(binding) {}
+
+  static auto classof(const Member* member) -> bool {
+    return member->Tag() == Kind::FieldMember;
+  }
+
+  auto Binding() const -> const BindingPattern* { return binding; }
+
+ private:
   // TODO: split this into a non-optional name and a type, initialized by
   // a constructor that takes a BindingPattern and handles errors like a
   // missing name.
   const BindingPattern* binding;
 };
 
-struct Member {
-  static auto MakeFieldMember(int line_num, const BindingPattern* binding)
-      -> Member*;
-
-  auto GetFieldMember() const -> const FieldMember&;
-
-  void Print(llvm::raw_ostream& out) const;
-  LLVM_DUMP_METHOD void Dump() const { Print(llvm::errs()); }
-
-  inline auto tag() const -> MemberKind {
-    return std::visit([](const auto& t) { return t.Kind; }, value);
-  }
-
-  int line_num;
-
- private:
-  std::variant<FieldMember> value;
-};
-
 }  // namespace Carbon
 
 #endif  // EXECUTABLE_SEMANTICS_AST_MEMBER_H_