[Flang] Missing diagnostic on duplicate defined I/O

[DanielCChen]

Consider the following code:

module m
    type base
        integer id

        contains

        generic :: write (formatted) => write
        procedure, private :: write => writeb
    end type

    abstract interface
        subroutine absWrite (dtv, unit, iotype, v_list, iostat, iomsg)
        import
            class(base), intent(in) :: dtv
            integer, intent(in) :: unit
            character(*), intent(in) :: iotype
            integer, intent(in)  :: v_list(:)
            integer, intent(out) :: iostat
            character(*), intent(inout) :: iomsg
        end subroutine
    end interface

    procedure(absWrite) writeb, writebaseext

    interface write(formatted)
        procedure writebaseext
    end interface
end module

end

Both writeb and writebaseext have identical interface for the same defined I/O for type base. This should be diagnosed as duplicate.

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: Daniel Chen (DanielCChen)

Consider the following code: ``` module m type base integer id

    contains

    generic :: write (formatted) => write
    procedure, private :: write => writeb
end type

abstract interface
    subroutine absWrite (dtv, unit, iotype, v_list, iostat, iomsg)
    import
        class(base), intent(in) :: dtv
        integer, intent(in) :: unit
        character(*), intent(in) :: iotype
        integer, intent(in)  :: v_list(:)
        integer, intent(out) :: iostat
        character(*), intent(inout) :: iomsg
    end subroutine
end interface

procedure(absWrite) writeb, writebaseext

interface write(formatted)
    procedure writebaseext
end interface

end module

end

Both `writeb` and `writebaseext` have identical interface for the same defined I/O for type `base`. This should be diagnosed as duplicate.
</details>

[eugeneepshteyn]

ifx 2025.0:

$ ifx -c defined-io-ambiguous.f90 
defined-io-ambiguous.f90(28): error #8638: The type/rank signature for arguments of this specific subroutine is identical to another specific subroutine that shares the same defined I/O.   [WRITEBASEEXT]
end module
^
compilation aborted for defined-io-ambiguous.f90 (code 1)

gfortran 14 seems to be complaining about something else:

$ gfortran -c defined-io-ambiguous.f90 
defined-io-ambiguous.f90:8:17:

    8 |         procedure, private :: write => writeb
      |                 1
Error: ‘writeb’ must be a module procedure or an external procedure with an explicit interface at (1)
defined-io-ambiguous.f90:8:45:

    8 |         procedure, private :: write => writeb
      |                                             1
Error: Too few dummy arguments in DTIO procedure ‘writeb’ at (1)

[klausler]

This compiler does catch this conflict when it processes the subroutine definitions, but it only works when they appear in the same source file (or maybe the same module). I'll change that check to cover this case as well.