Fortran standard and intrinsic procedure standard - fortran

Question

In the documentation of the GNU Fortran compiler, for a specific intrinsic function, there is a standard. For example, CPU_TIME is an intrinsic procedure for which it is indicated that the standard is Fortran 95 and later. My question is about the meaning of this.
I understand the Fortran standard to be like a set of rules to follow for your code to be defined as standard-compliant (to Fortran 77,90,95,2003 or 2008). Does this mean that CPU_TIME can only be used with Fortran 95+? Actually, I already know the answer, I can use CPU_TIME within a Fortran 77 file with the .f extension without the compiler to complain, compiling with a Gfortran version > 5. Is that because the compiler is able to treat any standard encountered in a code? I am aware of the -std flag but how can I be sure that for instance a Fortran 77 code uses only intrinsic procedures for Fortran 77?

Answer 1

Short answer, you can not distinguish standard below f95 with gnu compilers. From f95 and above, you can use the option -std to force the compiler to consider features of standard above as errors.
Long answer:
The documentation of gnu compiler says this:
-std=std
Specify the standard to which the program is expected to conform, which may be one of ‘f95’, ‘f2003’, ‘f2008’, ‘gnu’, or ‘legacy’. The
default value for std is ‘gnu’, which specifies a superset of the
Fortran 95 standard that includes all of the extensions supported by
GNU Fortran, although warnings will be given for obsolete extensions
not recommended for use in new code. The ‘legacy’ value is equivalent
but without the warnings for obsolete extensions, and may be useful
for old non-standard programs. The ‘f95’, ‘f2003’ and ‘f2008’ values
specify strict conformance to the Fortran 95, Fortran 2003 and Fortran
2008 standards, respectively; errors are given for all extensions
beyond the relevant language standard, and warnings are given for the
Fortran 77 features that are permitted but obsolescent in later
standards. ‘-std=f2008ts’ allows the Fortran 2008 standard including
the additions of the Technical Specification (TS) 29113 on Further
Interoperability of Fortran with C and TS 18508 on Additional Parallel
Features in Fortran.
- `-std=f95` will consider features above f95 as errors
- `-std=f2003` will consider features above f2003 as errors
- `-std=f2008` will consider features above f2008 as errors
- etc.
You might want to check with other compilers.
Easy verification:, compile the following program (courtesy of Fortran wiki) with and without the option -std=f95 and see what happens.
module class_Circle
implicit none
private
real :: pi = 3.1415926535897931d0 ! Class-wide private constant
type, public :: Circle
real :: radius
contains
procedure :: area => circle_area
procedure :: print => circle_print
end type Circle
contains
function circle_area(this) result(area)
class(Circle), intent(in) :: this
real :: area
area = pi * this%radius**2
end function circle_area
subroutine circle_print(this)
class(Circle), intent(in) :: this
real :: area
area = this%area() ! Call the type-bound function
print *, 'Circle: r = ', this%radius, ' area = ', area
end subroutine circle_print
end module class_Circle
program circle_test
use class_Circle
implicit none
type(Circle) :: c ! Declare a variable of type Circle.
c = Circle(1.5) ! Use the implicit constructor, radius = 1.5.
call c%print ! Call the type-bound subroutine
end program circle_test