stubgen creates duplicate class definitions

I need a stub for the serial package, and attempted to use stubgen to create it. The problem arises in the serialposix subpackage, which defines the PlatformSpecific class differently depending upon the platform it's running on. When stubgen processes this code, it discards all the conditional pieces, the end result being multiple definitions of the PlatformSpecific class, which, not surprisingly, mypy is very unhappy about.

Here's what the original code looks like (trimmed for compactness):

plat = sys.platform.lower()
if plat[:5] == 'linux':    # Linux (confirmed)  # noqa
    class PlatformSpecific(PlatformSpecificBase):
        …
elif plat == 'cygwin':       # cygwin/win32 (confirmed)
    class PlatformSpecific(PlatformSpecificBase):
        …
elif plat[:6] == 'darwin':   # OS X
    class PlatformSpecific(PlatformSpecificBase):
        …

and so on. You get the picture.

Running mypy after generating the stub gives:

stubs/serial/serialposix.pyi:24: error: Name 'PlatformSpecific' already defined on line 20
stubs/serial/serialposix.pyi:29: error: Name 'PlatformSpecific' already defined on line 20
stubs/serial/serialposix.pyi:37: error: Name 'PlatformSpecific' already defined on line 20
stubs/serial/serialposix.pyi:42: error: Name 'PlatformSpecific' already defined on line 20

Now, I can manually go in and edit the generated stub, commenting out the class definitions for platforms other than the one I am running on. But the problem is that I can't then check in that stub and expect mypy to run correctly on other platforms.

I don't think this is a typeshed issue, because I don't see a way that the stub can be written to work across platforms, which seems to be a problem inherent in the way that stubgen, and stubs in general, work. Unless I'm missing something (which is entirely possible), there is nothing in mypy that will allow this type of code to be stubbed successfully.

Start with stubgen and its handling of the platform-conditional definitions in serialposix; the generated stubs/serial/serialposix.pyi shows the duplicate PlatformSpecific declarations. Determine how platform-specific class definitions should be represented, then verify that the generated stub no longer produces duplicate names and that mypy accepts it across platforms.