Assignment interfaces with allocatable polymorphic variables in gfortran 5.5.0 & 8.1.0.

[Janus Weil]

Hi Ethan,

> However, on version 5.5, I get the following errors:
>
> OUTPUT 2
> =========
> $ gfortran --version
> GNU Fortran (Ubuntu 5.5.0-12ubuntu1) 5.5.0 20171010
>
> $ gfortran baz_module.f90 test_public_assignment.f90 -o test.out
> test_public_assignment.f90:13:0:
>
>  wrk = e     ! invoking foo
>  1
> Error: Assignment to an allocatable polymorphic variable at (1) is not yet
> supported
> test_public_assignment.f90:19:0:

Note that gfortran version 7 can also compile the program in its
original form (so you don't necessarily need version 8), but
unfortunately earlier versions fail.


> Now I did find some workaround by setting the default accessibility of the
> module to be public and explicitly declaring the complement of the set of
> procedures and variables that I wanted to be private. I'm not sure what
> gfortran 5.5.0 is doing to be honest, but the definition of the assignment
> can be used in the calling program. Commenting out lines 5,6 and commenting
> in lines 8,9 in baz_module.f90 gave me the following output on gfortran
> 5.5.0
>
> OUTPUT 3
> =========
>  Compiler version: GCC version 5.5.0 20171010
>  Compiler options: -mtune=generic -march=x86-64
>  d:    4.00000000       2.00000000       1.00000000
>  e:    8.00000000       6.00000000       4.00000000
>  f:    32.0000000       18.0000000       8.00000000
> =========
>
> So I have two questions for the gfortran community: 1) do you know why
> inverting the module accessibility causes these assignments to work in
> gfortran 5.5.0? It seems as if the assignment interface was made public
> somehow, but I'm not certain.

Exactly, the "public" statement makes everything in the module public
by default. The assignment operator is private otherwise.


> 2) can you think of any *clean* solutions to
> this problem? I'd love backward compatibility while not going against the
> recommended standard of private default accessibility.

A more reasonable approach might be to not make everything public, but
just the assignment interface:

public assignment(=)

HTH,
Janus