Question

Why does GCC not warn me when I forget to specify the return type of a interface function? For me this behavior seams unexpected. What do you say to this?

This is the test-program (uncoment Line 6 and it works as it should):

program returntest
implicit none
    interface
    function givehalf(Y)
      double precision :: Y
      !double precision :: givehalf !<-- Uncomment this line
    end function givehalf
    end interface
    double precision :: temp

temp=givehalf(5.151515d0)
print*, 'result=   ',temp

end program returntest

function givehalf(Y)
    implicit none
    double precision :: Y
    double precision :: givehalf

    print*, 'Y=        ',Y
    givehalf=Y/2.0d0
    print*, 'return Y/2',givehalf

    return
end function givehalf

The result is this:

user@bapf028dl:/media/disk> gfortran44 -Wall return-test.f90
user@bapf028dl:/media/disk> ./a.out
  Y=           5.1515149999999998     
  return Y/2   2.5757574999999999     
  result=     -1.0579199790954590     
user@bapf028dl:/media/disk> ifort return-test.f90
user@bapf028dl:/media/disk> ./a.out 
  Y=           5.15151500000000     
  return Y/2   2.57575750000000     
  result=      2.57575750350952 

Answer 1

This is not a bug. The interface body inside an interface block forms a separate scope, so you should include an implicit none statement there to prevent yourself from making such errors. Without it the implicit typing rules are in effect, so the function is expected to return a real.

interface
    function givehalf(Y)
        implicit none                 !<-- now you should get an error during compilation
        double precision :: Y
        !double precision :: givehalf !<-- Uncomment this line
    end function givehalf
end interface