[patch, fortran] PR 47359 - warnings for constant conversion

[Thomas Koenig]

Hello world,

please find attached a rather lengthy patch, which attempts to deal
with warnings for conversions in Fortran once and for all (one can
hope ... :-)

The patch has two parts.

One part moves all the checking for conversion into the
gfc_<type>2<type> functions.  The checks are made much more
extensive; for example, integer conversions to real are also
checked because they can change the value.

There is one change in behavior:  Previosly, when a value
which did not change on a narrowing conversion was detected
with -Wconversion, it was not reported with -Wconversion-extra,
but there was a warning if only -Wconversion-extra was specified.

This patch changes that behavior so that -Wconversion and
-Wconversion-extra emit the same warning if the value is changed,
but that -Wconversion-extra, if specified alone, will emit
a warning anyway.

The second part catches the case when the user supplies more
digits than appropriate for the number.  More often than not,
this is a KIND error.

For the second part (which can also be committed separately)
I am not sure if the -Wconversion-extra flag is the right one.
I shied away from specifying YANF (Yet Another New Flag), but
I could do so if the consensus is that this is better.  It
might also be possible to invent another name for this option,
and maybe enable this with -Wsurprising.

Regression-tested.  Comments?  OK for trunk?

	Thomas

2015-05-31  Thomas Koenig  <tkoenig@netcologne.de>

        PR fortran/47359
        * arith.c (eval_intrinsic_op): Set warn flag for
        gfc_type_convert_binary if -Wconversion or -Wconversion-extra
        are set.
        (wprecision_real_real): New function.
        (wprecision_int_real): New function.
        (gfc_int2int): If -fno-range-check and -Wconversion are specified
        and it is a narrowing conversion, warn.
        (gfc_int2real): If there is a change in value for the conversion,
        warn.
        (gfc_int2complex):  Likewise.
        (gfc_real2int): If there is a fractional part to the real number,
        warn with -Wconversion, otherwise warn with -Wconversion-extra.
        (gfc_real2real): Emit warning if the constant was changed by
        conversion with either -Wconversion or -Wconversion-extra.  With
        -Wconversion-extra, warn if no warning was issued earlier.
        (gfc_real2complex):  Likewise.
        (gfc_complex2int): For -Wconversion or -Wconversion-extra, if
        there was an imaginary part, warn; otherwise, warn for change in
        value.  Warn with -Wconversion-extra if no other warning was
        issued.
        (gfc_complex2real): For -Wconversion or -Wconversion-extra, if
        there was an imaginary part, warn; otherwise, warn for change in
        value. Warn with -Wconversion-extra if no other warning was
        issued.
        (gfc_complex2complex):  For -Wconversion, warn if the value of
        either the real or the imaginary part was changed.  Warn for
        -Wconversion-extra if no prior warning was issued.
        * expr.c (gfc_check_assign):  Remove check for change in value.
        * primary.c (match_real_constant): For -Wconversion-extra, check
        against a number in which the last non-zero digit has been
        replaced with a zero.  If the number compares equal, warn.

2015-05-31  Thomas Koenig  <tkoenig@netcologne.de>

        PR fortran/47359
        * gfortran.dg/array_constructor_type_17.f03: Adjust error message.
        * gfortran.dg/warn_conversion.f90: Add warning for change in value
        for assignment.
        * gfortran.dg/warn_conversion_3.f90: Add warnings.
        * gfortran.dg/warn_conversion_5.f90: New test.
        * gfortran.dg/warn_conversion_6.f90: New test.
        * gfortran.dg/warn_conversion_7.f90: New test.
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