[Flang] Incorrect diagnose on NULL(mold) actual to allocatable dummy

[DanielCChen]

Consider the following code:

module m
    type Base
        integer i
    end type

    type, extends(Base) :: Child
        integer j
    end type
end module

program argAssociation001
use m
    type(Child), allocatable :: c

    allocate(c, SOURCE=Child(2,-2))
    call sub1(null(c))

    contains
    subroutine sub1(arg)
        type(Child), allocatable :: arg
    end subroutine
end

Flang issues an error as

t2.f:16:15: error: A null pointer may not be associated with allocatable dummy argument 'arg=' without INTENT(IN)
      call sub1(null(c))

The code seems conforming to me as I can't find such constraint in the standard.
Both XLF and ifort compile the code successfully.

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: Daniel Chen (DanielCChen)

Consider the following code: ``` module m type Base integer i end type

type, extends(Base) :: Child
    integer j
end type

end module

program argAssociation001
use m
type(Child), allocatable :: c

allocate(c, SOURCE=Child(2,-2))
call sub1(null(c))

contains
subroutine sub1(arg)
    type(Child), allocatable :: arg
end subroutine

end

Flang issues an error as

t2.f:16:15: error: A null pointer may not be associated with allocatable dummy argument 'arg=' without INTENT(IN)
call sub1(null(c))

The code seems conforming to me as I can't find such constraint in the standard. 
Both XLF and ifort compile the code successfully. 
</details>

[llvmbot]

@llvm/issue-subscribers-bug

Author: Daniel Chen (DanielCChen)

Consider the following code: ``` module m type Base integer i end type

type, extends(Base) :: Child
    integer j
end type

end module

program argAssociation001
use m
type(Child), allocatable :: c

allocate(c, SOURCE=Child(2,-2))
call sub1(null(c))

contains
subroutine sub1(arg)
    type(Child), allocatable :: arg
end subroutine

end

Flang issues an error as

t2.f:16:15: error: A null pointer may not be associated with allocatable dummy argument 'arg=' without INTENT(IN)
call sub1(null(c))

The code seems conforming to me as I can't find such constraint in the standard. 
Both XLF and ifort compile the code successfully. 
</details>

[klausler]

That is a valid error. You can pass NULL as an actual argument when the corresponding dummy is a pointer, is optional, or is an INTENT(IN) allocatable. But it doesn't make sense to pass NULL for a dummy that is allocatable and neither optional nor INTENT(IN) -- there's no effective actual argument with which the dummy argument can be associated.

[DanielCChen]

@klausler Reopen for some clarification.
The standard says the following about the dummy argument.

The actual argument shall be allocatable. It is permissible for the actual argument to have an allocation status of unallocated.

For intrinsic NULL(MOLD).

Result Characteristics. If MOLD is present, the characteristics are the same as MOLD.
...
Result: The result is a disassociated pointer or an unallocated allocatable entity.

This is exactly this test case is testing. The actual argument is an unallocated allocatable object that corresponds to an allocatable dummy.

It initializes the dummy argument using NULL() intrinsic rather than deallocate c first then pass it.

[klausler]

But NULL(MOLD=c) is an expression, not a variable, so it is not possible for the actual argument to become defined. That's why I require the INTENT(IN). I should probably relax that to also allow this case of a dummy argument with no INTENT.

[DanielCChen]

@klausler Thanks for the fix.
I still think the "warning" message is bit confusing.

NULL(MOLD) is a unique case in the sense that the result is an ... unallocated allocatable entity according to the standard unlike a user defined function that returns an allocatable result. User function allocatable result is the value of it, and it doesn't have the allocatable attribute.

The message requires the dummy argument to have the INTENT(IN) attribute, which it makes the dummy argument useless as users cannot change its allocation status, which is unallocated.

Same for the case of call sub1(NULL()) without the MOLD argument as the standard says it would have the TKR of the corresponding dummy argument.

I think this is a legal case of using intrinsic NULL.