[PATCH] PR libstdc++/59568 fix error handling for std::complex stream extraction

[Jonathan Wakely <jwakely at redhat dot com>]

The bug here is that we called putback even if the initial __is >> __ch
extraction failed and set eofbit, and putback clears the eofbit. I
found a number of other problems though, such as not even trying to
call putback after failing to find the ',' and ')' characters.

I decided to rewrite the function following the proposed resolution
for https://wg21.link/lwg2714 which is a much more precise
specification for much more desirable semantics.

	PR libstdc++/59568
	* include/std/complex (operator>>): Implement proposed resolution to
	LWG 2714. Use putback if and only if a character has been successfully
	extracted but isn't a delimiter. Use ctype::widen and traits::eq when
	testing if extracted characters match delimiters.
	* testsuite/26_numerics/complex/dr2714.cc: New test.

Tested powerpc64le-linux, committed to trunk.

For the release branches I'm considering just fixing the bug that
clears eofbit, and not the whole rewrite of the function.

Attachment: patch.txt
Description: Text document