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From: Jonathan Wakely <jwakely.gcc@gmail.com>
Date: Thu, 1 Dec 2022 23:56:42 +0000
Message-ID: <CAH6eHdR7GHLH6PZZnyCJ=WqY25w5y43CZqrJD5KOVUq5ibDsDQ@mail.gmail.com>
Subject: Re: which compiler is right (either to compile or to barf)...
To: leon zadorin <leonleon77@gmail.com>
Cc: gcc-help <gcc-help@gcc.gnu.org>
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On Thu, 1 Dec 2022 at 23:54, Jonathan Wakely wrote:
> Those constructors are irrelevant. It's undefined to instantiate *any*
> std::lib template with an incomplete type unless *explicitly*
> permitted. It is explicitly permitted for vector and shared_ptr. It is
> not permitted for optional.

I forgot to give the standard reference. See [res.on.functions] p2 (2.5).