For the program

program main
implicit none
real :: xx
double precision :: yy
call random_seed()
call random_number(yy)
xx = yy ! assign double to single
print*,"yy =",yy
print*,"xx =",xx
end program main

g95 -Wall -Wextra says

In file xreal_dp.f90:7

xx = yy
     1
Warning (140): Implicit conversion at (1) may cause
precision loss

I request a similar warning from gfortran. None is currently given when compiling with 

gfortran -Wall -pedantic -std=f2003 xreal_dp.f90 .

Vivek Rao