Allow subclassing from ModuleType

There are still a couple subtle differences in how we track and
store the various module attributes.  For example, we show something
different for 'print ModuleType("a", "b")'.

Also sneak in a fix for "import a.b.c as c"

src/core/cfg.cpp

@@ -1303,8 +1303,9 @@ public:
                         l = r + 1;
                         continue;
                     }
-                    pushAssign(tmpname, new AST_Attribute(makeName(tmpname, AST_TYPE::Load, node->lineno),
-                                                          AST_TYPE::Load, internString(a->name.str().substr(l, r))));
+                    pushAssign(tmpname,
+                               new AST_Attribute(makeName(tmpname, AST_TYPE::Load, node->lineno), AST_TYPE::Load,
+                                                 internString(a->name.str().substr(l, r - l))));
                     l = r + 1;
                 } while (l < a->name.str().size());
                 pushAssign(a->asname, makeName(tmpname, AST_TYPE::Load, node->lineno));

src/runtime/types.cpp

@@ -918,8 +918,19 @@ Box* typeMro(BoxedClass* self) {
     return r;
 }
 
+Box* moduleNew(BoxedClass* cls, BoxedString* name, BoxedString* fn) {
+    RELEASE_ASSERT(isSubclass(cls, module_cls), "");
+    RELEASE_ASSERT(name->cls == str_cls, "");
+    RELEASE_ASSERT(!fn || fn->cls == str_cls, "");
+
+    if (fn)
+        return new (cls) BoxedModule(name->s, fn->s);
+    else
+        return new (cls) BoxedModule(name->s, "__builtin__");
+}
+
 Box* moduleRepr(BoxedModule* m) {
-    assert(m->cls == module_cls);
+    RELEASE_ASSERT(isSubclass(m->cls, module_cls), "");
 
     std::ostringstream os;
     os << "<module '" << m->name() << "' ";
@@ -1395,6 +1406,8 @@ void setupRuntime() {
     none_cls->giveAttr("__nonzero__", new BoxedFunction(boxRTFunction((void*)noneNonzero, BOXED_BOOL, 1)));
     none_cls->freeze();
 
+    module_cls->giveAttr("__new__",
+                         new BoxedFunction(boxRTFunction((void*)moduleNew, UNKNOWN, 3, 1, false, false), { NULL }));
     module_cls->giveAttr("__repr__", new BoxedFunction(boxRTFunction((void*)moduleRepr, STR, 1)));
     module_cls->freeze();
 

src/runtime/types.h

@@ -466,7 +466,9 @@ public:
 class BoxedModule : public Box {
 public:
     HCAttrs attrs;
-    std::string fn; // for traceback purposes; not the same as __file__
+
+    // for traceback purposes; not the same as __file__.  This corresponds to co_filename
+    std::string fn;
     FutureFlags future_flags;
 
     BoxedModule(const std::string& name, const std::string& fn);

test/tests/distutils_test.py

-# skip-if: True
-# - wip
 from distutils.core import setup
+
+import distutils.command.build as build

test/tests/module_subclassing.py

+from types import ModuleType
+
+class MyModule(ModuleType):
+    pass
+
+print MyModule('o')
+