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/interpreter/action_stack.h

@@ -15,16 +15,19 @@
 
 namespace Carbon {
 
+// Selects between compile-time and run-time behavior.
+enum class Phase { CompileTime, RunTime };
+
 // The stack of Actions currently being executed by the interpreter.
 class ActionStack {
  public:
   // Constructs an empty compile-time ActionStack.
-  ActionStack() = default;
+  ActionStack() : phase_(Phase::CompileTime) {}
 
   // Constructs an empty run-time ActionStack that allocates global variables
   // on `heap`.
   explicit ActionStack(Nonnull<HeapAllocationInterface*> heap)
-      : globals_(RuntimeScope(heap)) {}
+      : globals_(RuntimeScope(heap)), phase_(Phase::RunTime) {}
 
   void Print(llvm::raw_ostream& out) const;
   LLVM_DUMP_METHOD void Dump() const { Print(llvm::errs()); }
@@ -120,6 +123,7 @@ class ActionStack {
   Stack<std::unique_ptr<Action>> todo_;
   std::optional<Nonnull<const Value*>> result_;
   std::optional<RuntimeScope> globals_;
+  Phase phase_;
 };
 
 }  // namespace Carbon