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/impl_binding.h

@@ -0,0 +1,79 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef EXECUTABLE_SEMANTICS_AST_GENERIC_BINDING_H_
+#define EXECUTABLE_SEMANTICS_AST_GENERIC_BINDING_H_
+
+#include <map>
+
+#include "common/check.h"
+#include "common/ostream.h"
+#include "executable_semantics/ast/ast_node.h"
+#include "executable_semantics/ast/pattern.h"
+#include "executable_semantics/ast/value_category.h"
+
+namespace Carbon {
+
+class Value;
+class Expression;
+class ImplBinding;
+
+// The run-time counterpart of a `GenericBinding`.
+//
+// Once a generic binding has been declared, it can be used
+// in two different ways: as a compile-time constant with a
+// symbolic value (such as a `VariableType`), or as a run-time
+// variable with a concrete value that is stored on the stack.
+// An `ImplBinding` is used in contexts where the second
+// interpretation is intended.
+class ImplBinding : public AstNode {
+ public:
+  using ImplementsCarbonValueNode = void;
+
+  ImplBinding(SourceLocation source_loc,
+              Nonnull<const GenericBinding*> type_var,
+              Nonnull<const Value*> iface)
+      : AstNode(AstNodeKind::ImplBinding, source_loc),
+        type_var_(type_var),
+        iface_(iface) {}
+
+  static auto classof(const AstNode* node) -> bool {
+    return InheritsFromImplBinding(node->kind());
+  }
+  void Print(llvm::raw_ostream& out) const override;
+  void PrintID(llvm::raw_ostream& out) const override;
+
+  // The binding for the type variable.
+  auto type_var() const -> Nonnull<const GenericBinding*> { return type_var_; }
+  // The interface being implemented.
+  auto interface() const -> Nonnull<const Value*> { return iface_; }
+
+  // Required for the the ValueNode interface
+  auto constant_value() const -> std::optional<Nonnull<const Value*>> {
+    return std::nullopt;
+  }
+  auto symbolic_identity() const -> std::optional<Nonnull<const Value*>> {
+    return std::nullopt;
+  }
+
+  // The static type of the impl. Cannot be called before typechecking.
+  auto static_type() const -> const Value& { return **static_type_; }
+
+  // Sets the static type of the impl. Can only be called once, during
+  // typechecking.
+  void set_static_type(Nonnull<const Value*> type) {
+    CHECK(!static_type_.has_value());
+    static_type_ = type;
+  }
+  auto value_category() const -> ValueCategory { return ValueCategory::Let; }
+
+ private:
+  Nonnull<const GenericBinding*> type_var_;
+  Nonnull<const Value*> iface_;
+  std::optional<Nonnull<const Value*>> static_type_;
+};
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_AST_GENERIC_BINDING_H_