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[PATCH] PR libstdc++/87431 optimise valueless_by_exception()

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: Tue, 25 Sep 2018 15:59:07 +0100
Subject: [PATCH] PR libstdc++/87431 optimise valueless_by_exception()

If a std::variant can never get into valueless state then we don't need
to do a runtime check for a valid alternative.

	PR libstdc++/87431
	* include/std/variant (_Variant_storage<true, _Types...>::_M_valid):
	Avoid runtime test when all alternatives are scalars and so cannot
	throw during initialization.

Tested x86_64-linux, committed to trunk.

Attachment: patch.txt
Description: Text document

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