Ensure all NamedEntities have static types (#968)

executable_semantics/ast/declaration.h

@@ -91,9 +91,22 @@ struct GenericBinding : public virtual AstNode, public NamedEntity {
   auto type() const -> const Expression& { return *type_; }
   auto type() -> Expression& { return *type_; }
 
+  // The static type of the binding. Cannot be called before typechecking.
+  auto static_type() const -> const Value& { return **static_type_; }
+
+  // Sets the static type of the binding. Can only be called once, during
+  // typechecking.
+  void set_static_type(Nonnull<const Value*> type) { static_type_ = type; }
+
+  // Returns whether the static type has been set. Should only be called
+  // during typechecking: before typechecking it's guaranteed to be false,
+  // and after typechecking it's guaranteed to be true.
+  auto has_static_type() const -> bool { return static_type_.has_value(); }
+
  private:
   std::string name_;
   Nonnull<Expression*> type_;
+  std::optional<Nonnull<const Value*>> static_type_;
 };
 
 // The syntactic representation of a function declaration's return type.