The Style Guide currently says:

  Put a member function `FooBar::bang` into the same file that
  defined `FooBar`, or its associated *.cpp file.

It probably shouldn't promote putting the implementation in the header. But there is also this:

  * Do not put non-trivial function implementations in .hpp files. If
  the implementation depends on other .hpp files, put it in a .cpp or
  a .inline.hpp file.

However, the trend toward lots of .inline.hpp files is not necessarily good either, especially since we may not be using them properly. But that may be a separate discussion.

Probably those two quoted sections should be merged or linked in some way, in additional to any content changes.