[PATCH] PR libstdc++/83982 fix exception-safety guarantee of std::vector::resize

[Jonathan Wakely <jwakely at redhat dot com>]

Construct new elements before moving existing ones, so that if a default
constructor throws, the existing elements are not left in a moved-from
state.

2018-06-14  Daniel Trebbien <dtrebbien@gmail.com>
	    Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/83982
	* include/bits/vector.tcc (vector::_M_default_append(size_type)):
	Default-construct new elements before moving existing ones.
	* testsuite/23_containers/vector/capacity/resize/strong_guarantee.cc:
	New.

Tested powerpc64le-linux.

Daniel (CC'd) originally proposed a fix based on the code used in
vector::_M_realloc_insert. This is a slightly simpler fix, possible
because resize only inserts at the end not in the middle. I also wrote
a new testcase which is also valid in C++98, where the lack of move
constructors means the strong exception-safety guarantee is always
met. (Which means the bug is a regression since the default mode
changed to -std=gnu++14 in GCC 6.1).

I plan to commit this to trunk tomorrow, and backport soon too.

Attachment: patch.txt
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