[Bug fortran/69368] [6 Regression] spec2006 test case 416.gamess fails with the g++ 6.0 compiler starting with r232508

[alalaw01 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69368

--- Comment #79 from alalaw01 at gcc dot gnu.org ---
(In reply to rguenther@suse.de from comment #78)
>
> That would pessimize it too much IMHO.

I'm not sure how to evaluate the pessimization, given it's thought to be a
widespread pseudo-FORTRAN construct; so I probably have to defer to your
judgement here. However...

Given maxsize of an array as two elements, say, would the compiler not be
entitled to optimize an index selection down to, say, computing only the LSBit
of the actual index?  Whereas 'unknown' means, well, exactly what is the case.
So I fear this is storing problems up for the future.

Is the concern that we can't hide this behind an option, as that would "drive
people away from gfortran" ? If that's the case, can we hide it behind an
option that defaults to pessimization (?? at least for fortran)??