https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101209
--- Comment #1 from Aaron Ballman <aaron at aaronballman dot com> --- > This should likely give an error about using a trailing return type for a > function with a declared return type other than a placeholder type. Err, that would be bad now that I think about it (http://eel.is/c++draft/class.conv.fct#6). So I'd amend this to: it should probably give a reasonable diagnostic. :-)