[Bug fortran/80701] New: gfortran ignores dead code after return statement

["gustavo.hime at mpimet dot mpg.de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80701

            Bug ID: 80701
           Summary: gfortran ignores dead code after return statement
           Product: gcc
           Version: 6.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gustavo.hime at mpimet dot mpg.de
  Target Milestone: ---

Created attachment 41336
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41336&action=edit
bug.f90

Compiling the attached code with "gfortran -c bug.f90" generates no warnings.
However, if one inspects the object file, there is no mention of the
"undefined" symbol that is used in the code. If one comments out the RETURN
statement, it still compiles without warnings, but the symbol is there. This
behaviour happens independently of the value of -On, both on 5.2.0 and 6.2.0.

For the record, NAG and Intel behave similarly, but I would like to have the
compiler throw at least warnings for unreachable code.

The compiler does generate a warning about the implicit interface if
-Wimplicit-interface is turned on explicitly. I would suggest this should be on
by default with -Wall on any source not written in F77, or when a higher
standard is enforced.