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/syntax/parser.ypp

@@ -159,6 +159,7 @@
 %type <Nonnull<Pattern*>> paren_pattern
 %type <Nonnull<TuplePattern*>> tuple_pattern
 %type <Nonnull<TuplePattern*>> maybe_empty_tuple_pattern
+%type <std::optional<Nonnull<TuplePattern*>>> type_params
 %type <ParenContents<Pattern>> paren_pattern_base
 %type <ParenContents<Pattern>> paren_pattern_contents
 %type <Nonnull<AlternativeSignature*>> alternative
@@ -580,6 +581,8 @@ non_expression_pattern:
       $$ = arena->New<BindingPattern>(context.source_loc(), $1, $3,
                                       std::nullopt);
     }
+| binding_lhs COLON_BANG expression
+    { $$ = arena->New<GenericBinding>(context.source_loc(), $1, $3); }
 | paren_pattern
     { $$ = $1; }
 | postfix_expression tuple_pattern
@@ -868,11 +871,17 @@ alternative_list_contents:
       $$.push_back(std::move($3));
     }
 ;
+type_params:
+  // Empty
+    { $$ = std::nullopt; }
+| tuple_pattern
+    { $$ = $1; }
+;
 declaration:
   function_declaration
     { $$ = $1; }
-| CLASS identifier LEFT_CURLY_BRACE declaration_list RIGHT_CURLY_BRACE
-    { $$ = arena->New<ClassDeclaration>(context.source_loc(), $2, $4); }
+| CLASS identifier type_params LEFT_CURLY_BRACE declaration_list RIGHT_CURLY_BRACE
+    { $$ = arena->New<ClassDeclaration>(context.source_loc(), $2, $3, $5); }
 | CHOICE identifier LEFT_CURLY_BRACE alternative_list RIGHT_CURLY_BRACE
     { $$ = arena->New<ChoiceDeclaration>(context.source_loc(), $2, $4); }
 | VAR variable_declaration SEMICOLON