[PATCH] PR libstdc++/79206 check string_view sizes in operator==

[Jonathan Wakely <jwakely at redhat dot com>]

I've just committed this, and then noticed that we don't do the same
optimization for basic_string unless the char_type is char. Presumably
this is so that we do call basic_string::compare() and so call any
user-defined traits_type::eq() function (which is observable). I don't
think that's necessary, because the standard says operator== on
strings returns the result of lhs.compare(rhs) == 0, not that it's
"Equivalent to" such a call. As long as we give the right answer, I
don't think we need to make an explicit call to
basic_string::compare(). So maybe we want this for basic_string too.

	PR libstdc++/79206
	* include/experimental/string_view (operator==): Check sizes first.
	* include/std/string_view (operator==): Likewise.

Tested powerpc64le-linux, committed to trunk.

Attachment: patch.txt
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