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From: "mscfd at gmx dot net" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug fortran/97612] Structure constructor of type with nested
 allocatable array components fails to compile
Date: Mon, 07 Dec 2020 09:23:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97612

--- Comment #2 from martin <mscfd at gmx dot net> ---
I do not see why it should not be valid.
t() is a structure constructor acting much like a function with (in this ca=
se)
one optional argument. And as an allocatable component has default
initialisation, no value needs to be provided, to be valid constructor.

Just to be sure I checked Metcalf (2011). Section 15.3 "Structure construct=
ors"
and 20.1.4 "Structure constructors" contain an interesting piece of informa=
tion
I did not know about: F03 forgot to allow that for allocatable components a
value can be omitted. This was permitted in F08. So it looks like the provi=
ded
code example is not valid F03 but valid F08.

In fact, null() can be provided as default value. Then the code compiles:

type(t) :: a =3D t(null())=