I do not see the warning message (probably different
configuration file settings).  But I do see a variant
of the problem, where the documentation for Foo::u
does not appear in the HTML output.  In the example
above, the "/// foo" comment should have generated
HTML output, but it silently disappears.

I have C code, not C++, but the issue is the same.
I am using 1.8.3.1.

Probably configuration file settings matter. To produce the issue I've set JAVADOC_AUTOBRIEF and EXTRACT_PRIVATE.

Created attachment 243767 [details]
doxyfile used

(In reply to comment #0)
> Example:
> 
> /// class
> class Foo {
> public:
> private:
>     /// foo
>     union {
>         double d;
>         int i;
>     } u;
> };
> 
> This generates the warning: Member u (variable) of class Foo is not documented.
> 
> IIRC, this did not happen in release 1.8.1.2.

I found out that the member u can be documented by putting the doc comment in the "correct" line. This does what I want:

/// class
class Foo {
public:
private:
    union {
        double d;
        int i;
    /// foo
    } u;
};

I just checked and since version 1.8.4 either place (before the union or before the variable name) produces the correct results, so this bug seems to be fixed.

Please reopen this bug report if you still see this issue with the latest version of doxygen. Add a self-contained example (source+config file in a zip or tar) if possible.