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[PATCH] Implement LWG 3255 for std::span constructors

From: Jonathan Wakely <jwakely at redhat dot com>
To: libstdc++ at gcc dot gnu dot org, gcc-patches at gcc dot gnu dot org
Date: Mon, 30 Sep 2019 12:51:45 +0100
Subject: [PATCH] Implement LWG 3255 for std::span constructors

Also fix the constraints on span(Container&) and span(const Container&)
constructors so that they aren't used for const spans or const arrays.

	* include/std/span (span(element_type(&)[N]))
	(span(array<value_type, N>&), span(const array<value_type, N>&)):
	Deduce array element type to allow safe const conversions (LWG 3255).
	[!_GLIBCXX_P1394] (span(Container&), span(const Container&)): Use
	remove_cv_t on arguments to __is_std_span and __is_std_array.
	* testsuite/23_containers/span/lwg3255.cc: New test.

This library issue hasn't been resolved yet, but the proposed
resolution looks right and so I'm implementing it now.

Tested powerpc64le-linux, committed to trunk.

Attachment: patch.txt
Description: Text document

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