[Patch, Fortran] PR 82018: -Wextra should imply -Wconversion-extra

[Thomas Koenig]

Am 18.09.2017 um 11:50 schrieb Dominique d'Humières:
> Warning: Conversion from 'REAL(4)' to 'REAL(8)' at (1) [-Wconversion-extra]

Not me (not in the general case)

> even if may allow to detect things such as ‘pi8=acos(-1.0)’?

This one would be interesting to catch (even with -Wall), although
it would be hard to avoid false positives.

I agree with Dominique that enabling -Wconversion-extra
the way it is now would be too noisy.

Are there warnings currently enabled with -Wconversion-extra
which would be useful in -Wextra (or even ones currently not in
-Wconversion-extra) without having too many false positives?

r8 = 1./3. could be one example, r8=acos(-1.0) another.  Something
like "it must have been calculated with a formula and, if evaluated
in the precision of the lhs, gives a different result from the one
if the user had specified it with the correct precision".

We don't do that yet, but I think it could be useful even with -Wall,
and certainly with -Wextra.

What do you think?