[Bug fortran/99761] New: Warn flag for non-kind specified mixing

["nickpapior at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99761

            Bug ID: 99761
           Summary: Warn flag for non-kind specified mixing
           Product: gcc
           Version: fortran-dev
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nickpapior at gmail dot com
  Target Milestone: ---

Some context here:
https://github.com/j3-fortran/fortran_proposals/issues/201

The basic message is that users may forget to add kind specifications for
constants which are assigned to higher precision values which then loses the
extra bits.

Simplest example:

real(8), parameter :: pi = 3.14159265358979323846

which actually only stores the floating point value and store that in a double
precision variable, thereby loosing precision.

My proposal would be to add a flag which warns about misused kinds:

program test
  real(8), parameter :: const = 1.4435435345345
  real(8), parameter :: const2 = 1./3.
  real(8), parameter :: const3 = 1._4/4._4
  print *, 1./3. * const
end program test

it would be nice if the above could be compiled with gfortran -Wpedantic-kind
and something like this would be issued:


    2 |    real(8), parameter :: const = 1.4435435345345
                                         1
Warning: Constant expression at (1) is in lower precision than variable.

    3 |    real(8), parameter :: const2 = 1./3.
                                          1
Warning: Constant expression at (1) is in lower precision than variable.

    3 |    real(8), parameter :: const2 = 1./3.
                                             1
Warning: Constant expression at (1) is in lower precision than variable.


    5 |    print *, 1./3. * const
                     1      2
Warning: Constant at (1) has lower precision than variable at (2)


Line 4  is silently ignored due to explicit kind specification.

I think such an enhancement would be extremely useful to hunt down mis-typed
kind specifiers.