[translation] C+ type stubs for frontend/args_def.py.

Down to 3 compile errors with frontend/args.py!  All related to
args::UsageError().

cpp/frontend_arg_def.h

@@ -0,0 +1,34 @@
+// frontend_arg_def.h
+
+#ifndef FRONTEND_ARG_DEF_H
+#define FRONTEND_ARG_DEF_H
+
+#include "mylib.h"
+
+namespace args {
+class _Action;
+class SetToArg;
+};
+
+namespace arg_def {
+
+class _FlagSpec {
+ public:
+  Dict<Str*, bool>* arity0;
+  Dict<Str*, args::SetToArg*>* arity1;
+  Dict<Str*, bool>* options;
+  Dict<Str*, runtime_asdl::value_t*>* defaults;
+};
+
+class _FlagSpecAndMore {
+ public:
+  Dict<Str*, args::_Action*>* actions_long;
+  Dict<Str*, args::_Action*>* actions_short;
+  Dict<Str*, runtime_asdl::value_t*>* defaults;
+
+};
+
+}  // namespace arg_def
+
+#endif  // FRONTEND_ARG_DEF_H
+

cpp/preamble.h

@@ -15,6 +15,7 @@ using id_kind_asdl::Kind_t;
 
 // oil/cpp
 #include "core_error.h"
+#include "frontend_arg_def.h"
 #include "frontend_match.h"
 #include "frontend_tdop.h"
 #include "osh_arith_parse.h"

frontend/args.py

@@ -66,11 +66,6 @@
 from mycpp import mylib
 from mycpp.mylib import tagswitch, iteritems
 
-try:
-  import libc  # OilFlags uses regexes right now.
-except ImportError:  # circular dependecy with arg_gen
-  libc = None
-
 from typing import (
     cast, Tuple, Optional, Dict, List, Any, IO, TYPE_CHECKING
 )
@@ -217,7 +212,8 @@ def Peek2(self):
     None is your SENTINEL for parsing.
     """
     if self.i >= self.n:
-      return None, -1
+      no_str = None  # type: str
+      return no_str, -1
     else:
       return self.argv[self.i], self.spids[self.i]
 
@@ -615,6 +611,11 @@ def ParseMore(spec, arg_r):
 
 
 if mylib.PYTHON:
+  try:
+    import libc  # OilFlags uses regexes right now.
+  except ImportError:  # circular dependecy with arg_gen
+    libc = None
+
   def ParseOil(spec, arg_r):
     # type: (arg_def._OilFlags, Reader) -> Tuple[_Attributes, int]
     out = _Attributes(spec.defaults)

osh/tdop.py

@@ -149,6 +149,7 @@ def LeftAssign(p, w, left, rbp):
 
 #
 # Parser definition
+# TODO: To be consistent, move this to osh/tdop_def.py.
 #
 
 if mylib.PYTHON: