[Bug c++/55077] implement and enable by default -Wliteral-conversion

["dcb314 at hotmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55077

--- Comment #6 from David Binderman <dcb314 at hotmail dot com> ---
(In reply to Manuel LÃpez-IbÃÃez from comment #5)
> Created attachment 33637 [details]
> untested patch
> 
> Untested patch. Bonus points if we show the value before and after
> conversion like clang does.

I tried out the patch by building a test compiler and
then building the current Linux kernel. 

Trivial problem with doubled tokens (&& &&), but the main problem is that the
patch doesn't fix the problem I described of double and float literals
into integral types.

Also, plenty of false positives for integer into smaller integral types,
for example integer into short or integer into char. 

For example, the patch warns for this reasonable code:

 charVariable = 0x80;

As is, I would not use the patch, but maybe with some further work,
it might be suitable for future use.