compiler flag for "preview all"

[Bruce Carneal]

One of the ideas floated at beerconf was that of a single 
compiler flag activating all "preview" options.  For reference 
there are 11 preview options available to the ldc user currently, 
and the set changes over time so.  A "preview all" flag would be 
very useful to anyone wanting to test as far in to the 
foreseeable future as possible.

The ldc and dmd documentation say that the preview=all flag will 
list all available preview options, but in my testing both ldc 
1.22.0 and dmd 2.091.1 treat preview=all as an error.  They *do* 
list the preview options without error when given preview=?.  
IOW, the preview=all formulation could be used for conveniently 
peering in to the future. At beerconf Iain indicated that he may 
have already implemented the capability on a private copy so gdc 
would also be a go if I read him correctly.

So, any objections to preview=all meaning "enable all known 
preview items"?  If not, I'd ask that one of the compiler command 
line maintainers make the request for the change or let me know 
how I might do so.

PS: Also at beerconf Stefan Koch opined that turning on all 
preview options might not be possible due to incompatibilities of 
some sort which, if true, would be an excellent reason for adding 
the capability.