When an annotation processor is working with a file - say, a resource found in the source location - and calls FileObject.toUri on it, normally (assuming #6419926 is fixed) a file:/... URI is returned which can then be used to refer to that resource, say by passing it to an XML parser.

When however the source tree is located on a Windows UNC share (\\server\share\path\...), the returned URI is of the form

  file:////server/share/path/...

This has well-known problems documented in #4723726; namely, the normalized form is

  file:/server/share/path/...

which translates to a different (likely nonexistent) file path.

The desired form is that specified by Microsoft:

  file://server/share/path/...

(Of course other code such as XML parsers then needs to handle proper UNC URIs as well, but that is outside the scope of the compiler.)