[Bug target/108742] Incorrect constant folding with (or exposed by) -fexcess-precision=standard

["matz at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108742

--- Comment #3 from Michael Matz <matz at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #2)
> Note, internally in standard excess precision, 4.2 seen by the lexer is
> actually
> EXCESS_PRECISION <double, 4.2L>,

Then _that_ is the problem.  The literal "4.2" simply is not a long double
literal "4.2L".

> when it is assigned to a double variable or
> cast
> to double (i.e. in places where C/C++ require the excess precision to be
> converted to the narrower one) it is rounded to double,
> but when used as (long double)4.2 it is the same as 4.2L

I disagree.  As "4.2" is "(double)4.2" then therefore "(long double)4.2" should
be the same as "(long double)(double)4.2".

> and even (long double)d == (long double)4.2 should behave
> the same as (long double)d == 4.2 and d == 4.2.

On this we agree, all these expressions should behave the same.  But I say they
should _not_ behave the same as "(long double)d == 4.2L".