[flang] Incorrect diagnostic on NULL intrinsic

[DanielCChen]

Consider the following code:

module m
    type base
        real(4) data
    end type

    interface operator (==)
        logical(4) function equal (b1, b2)
        import
            class (base), pointer, intent(in) :: b1, b2
        end function
    end interface
end module

program fclass022
use m
    class(base), pointer :: b1, b2

    nullify (b1, b2)

    if (b1 == null(b1)) ERROR STOP 3
end

Flang complains:

error: Semantic errors in t.f
./t.f:20:9: error: A NULL() pointer is not allowed as an operand here
      if (b1 == null(b1)) ERROR STOP 3
          ^^^^^^^^^^^^^^
./t.f:20:15: error: NULL() may not be used as an expression in this context
      if (b1 == null(b1)) ERROR STOP 3
                ^^^^^^^^

All gfortran, ifort and XLF accepts the code.

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: Daniel Chen (DanielCChen)

Consider the following code: ``` module m type base real(4) data end type

interface operator (==)
    logical(4) function equal (b1, b2)
    import
        class (base), pointer, intent(in) :: b1, b2
    end function
end interface

end module

program fclass022
use m
class(base), pointer :: b1, b2

nullify (b1, b2)

if (b1 == null(b1)) ERROR STOP 3

end

Flang complains:

error: Semantic errors in t.f
./t.f:20:9: error: A NULL() pointer is not allowed as an operand here
if (b1 == null(b1)) ERROR STOP 3
^^^^^^^^^^^^^^
./t.f:20:15: error: NULL() may not be used as an expression in this context
if (b1 == null(b1)) ERROR STOP 3
^^^^^^^^

All gfortran, ifort and XLF accepts the code.
</details>

[klausler]

If you're trying to compare pointers, that's not how it's done in Fortran. This program will dereference the NULL pointer at execution time -- or it would, if I didn't emit a compilation-time error message as above.

[DanielCChen]

This is an user-defined operator so the compiler should not fail it at the compile time. I updated the test case with the definition of the user-defined operator in the test case. It will check the association status to determine the result.