doxygen message I don't understand

I am running debian stable.

doxygen is emitting this message:

----

/home/n7dr/projects/drlog/src/drlog_context.cpp:1615: warning: documented
symbol `decltype' was not declared or defined.

----

This is the relevant part of the source file:

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/*! \brief      Get names and values of sent exchange fields for a particular mode
    \param  m   target mode
    \return     the names and values of all the fields in the sent exchange
when the mode is <i>m</i>
*/
const decltype(drlog_context::_sent_exchange)
drlog_context::sent_exchange(const MODE m)

----

I don't understand what doxygen is complaining about. It looks fine to me, and
there is no problem compiling. So how do I get rid of the message from doxygen?

  Doc

-- 
Web:  http://enginehousebooks.com/drevans

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