If a .cpp file has no associated .h file, it will not #include it. However, check-webkit-style will report an error: "Found other header before a header this file implements. Should be: config.h, primary header, blank line, and then alphabetically sorted. [build/include_order] [4]"
This came up again while creating an all-in-one file for Windows: https://bugs.webkit.org/show_bug.cgi?id=80346