Generic classes (#1124)

* start of generic classes

* fix regressions

* class functions in interfaces

* access to class function on interface from class parameter

* more stuff working for generic classes

* fixing bugs

* update TypeEqual for generic classes

* fixing bugs and finding new ones

* disable unqualified access to members from other members for now

* minor edits

* bug fixes

* introduce compile_time_value to use in type checker instead of constant_value

* cleanup

* put a CHECK back in

* failure test cases for the new FATAL_COMPILATION_ERROR

* change a runtime FATAL into a FATAL_COMPILATION_ERROR

* Update executable_semantics/ast/declaration.h

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/action_stack.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/interpreter.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/interpreter.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/resolve_names.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* responses to reviews

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* rename compile_time_value to symbolic_identity

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* more edits from review

* Apply suggestions from code review

Co-authored-by: Geoff Romer <gromer@google.com>

* review responses

* Update executable_semantics/interpreter/interpreter.cpp

Co-authored-by: Geoff Romer <gromer@google.com>

* const impl_scope for TypeCheckChoiceDeclaration

* add some const

Co-authored-by: Jon Meow <jperkins@google.com>
Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/ast/statement.h

@@ -29,6 +29,7 @@ class Statement : public AstNode {
   ~Statement() override = 0;
 
   void Print(llvm::raw_ostream& out) const override { PrintDepth(-1, out); }
+  void PrintID(llvm::raw_ostream& out) const override { PrintDepth(1, out); }
   void PrintDepth(int depth, llvm::raw_ostream& out) const;
 
   static auto classof(const AstNode* node) {
@@ -350,6 +351,9 @@ class Continuation : public Statement {
   auto constant_value() const -> std::optional<Nonnull<const Value*>> {
     return std::nullopt;
   }
+  auto symbolic_identity() const -> std::optional<Nonnull<const Value*>> {
+    return std::nullopt;
+  }
 
  private:
   std::string name_;