The ddox documentation has some functions (see [1] and [2]) with return values of "auto auto". However, this does not happen with the other documentation (see [3] and [4], respectively). 

While I noticed this issue within the std.conv package, it is not limited to that and a google search for "auto auto" site:https://dlang.org/library/ reveals 220 results. Looking at a few of these issues, it appears that this happens when there are no attributes in the functions.  I did not see an example of this when it is a non-templated function, but I did not look that hard. I did not see the issue when looking at other functions in phobos that have attributes.

[1] https://dlang.org/library/std/conv/unsigned.html
[2] https://dlang.org/library/std/conv/signed.html
[3] https://dlang.org/phobos/std_conv.html#unsigned
[4] https://dlang.org/phobos/std_conv.html#signed