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/static_scope.h

@@ -38,15 +38,22 @@ static constexpr bool ImplementsValueNode = false;
   with a value, such as declarations and bindings. The interface consists of
   the following methods:
 
+  // Returns the constant associated with the node.
+  // This is called by the interpreter, not the type checker.
+  auto constant_value() const -> std::optional<Nonnull<const Value*>>;
+
+  // Returns the symbolic compile-time identity of the node.
+  // This is called by the type checker, not the interpreter.
+  auto symbolic_identity() const -> std::optional<Nonnull<const Value*>>;
+
   // Returns the static type of an IdentifierExpression that names *this.
   auto static_type() const -> const Value&;
 
   // Returns the value category of an IdentifierExpression that names *this.
   auto value_category() const -> ValueCategory;
 
-  // Print the node for diagnostic or tracing purposes.
-  void Print(llvm::raw_ostream& out) const;
-
+  // Print the node's identity (e.g. its name).
+  void PrintID(llvm::raw_ostream& out) const;
 
 */
 // TODO: consider turning the above documentation into real code, as sketched
@@ -70,9 +77,13 @@ class ValueNodeView {
             [](const AstNode& base) -> std::optional<Nonnull<const Value*>> {
               return llvm::cast<NodeType>(base).constant_value();
             }),
+        symbolic_identity_(
+            [](const AstNode& base) -> std::optional<Nonnull<const Value*>> {
+              return llvm::cast<NodeType>(base).symbolic_identity();
+            }),
         print_([](const AstNode& base, llvm::raw_ostream& out) -> void {
           // TODO: change this to print a summary of the node
-          return llvm::cast<NodeType>(base).Print(out);
+          return llvm::cast<NodeType>(base).PrintID(out);
         }),
         static_type_([](const AstNode& base) -> const Value& {
           return llvm::cast<NodeType>(base).static_type();
@@ -94,6 +105,11 @@ class ValueNodeView {
     return constant_value_(*base_);
   }
 
+  // Returns node->symbolic_identity()
+  auto symbolic_identity() const -> std::optional<Nonnull<const Value*>> {
+    return symbolic_identity_(*base_);
+  }
+
   void Print(llvm::raw_ostream& out) const { print_(*base_, out); }
 
   // Returns node->static_type()
@@ -123,6 +139,8 @@ class ValueNodeView {
   Nonnull<const AstNode*> base_;
   std::function<std::optional<Nonnull<const Value*>>(const AstNode&)>
       constant_value_;
+  std::function<std::optional<Nonnull<const Value*>>(const AstNode&)>
+      symbolic_identity_;
   std::function<void(const AstNode&, llvm::raw_ostream&)> print_;
   std::function<const Value&(const AstNode&)> static_type_;
   std::function<ValueCategory(const AstNode&)> value_category_;