[Bug fortran/113165] New: Code containing more than one type declaration for a variable results in confusing error messages from compiler

[xecej4 at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113165

            Bug ID: 113165
           Summary: Code containing more than one type declaration for a
                    variable results in confusing error messages from
                    compiler
           Product: gcc
           Version: 11.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xecej4 at outlook dot com
  Target Milestone: ---

Created attachment 56960
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56960&action=edit
Fixed form Fortran77 source file that causes compiler to issue confusing
messages

Some old Fortran codes (e.g., www.netlib.org/ode/epsode.f ) may contain
duplicate declarations of some variables. Lines 1239-1241 of epsode.f:

      double precision eps, epsj, h, hmax, hmin, pw, save1, save2,
     1                 ss, t, uround, ymax
      double precision d, r, r0, t, yj

The variable t is declared twice, albeit with the same type and kind.

Gfortran correctly flags this error, but seems to go farther and mark the
variable t as implicitly typed single precision REAL, causing the issuing a
number of misleading error messages when the concerned variable is used in
expressions later on in the program unit.

I enclose a short program to illustrate. Please compile the program with (a) no
options, (b) with -Wall and (c) with -fimplicit-none. Case (c) results in:

muldecl.f:4:24:

    4 |       double precision d,q,r  ! 'd' was already declared as double
prec.
      |                        1
Error: Symbol 'd' at (1) already has basic type of REAL
muldecl.f:7:7:

    7 |       r = sqrt(d)
      |       1
Error: Symbol 'r' at (1) has no IMPLICIT type

I suspect that the multiple declaration of d has caused even the unrepeated
declaration of r to be tainted.