[PATCH] libstdc++: Fix reverse iteration in _Utf16_view

[Jonathan Wakely]

When iterating over a range of char16_t in reverse the _Utf_view was
incorrectly treating U+DC00 as a valid high surrogate that can precede
the low surrogate. But U+DC00 is a low surrogate, and so should not be
allowed before another low surrogate. The check should be u2 >= 0xDC00
rather than u2 > 0xDC00.

libstdc++-v3/ChangeLog:

	* include/bits/unicode.h (_Utf_view::_M_read_reverse_utf16):
	Fix check for high surrogate preceding low surrogate.
	* testsuite/ext/unicode/view.cc: Check unpaired low surrogates.
---

Tested x86_64-linux.

 libstdc++-v3/include/bits/unicode.h        | 2 +-
 libstdc++-v3/testsuite/ext/unicode/view.cc | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/unicode.h b/libstdc++-v3/include/bits/unicode.h
index 00efbe89ca8e..44872db4ed68 100644
--- a/libstdc++-v3/include/bits/unicode.h
+++ b/libstdc++-v3/include/bits/unicode.h
@@ -527,7 +527,7 @@ namespace __unicode
 	  {
 	    // read a low surrogate, expect a high surrogate before it.
 	    uint16_t __u2 = *--_M_curr();
-	    if (__u2 < 0xD800 || __u2 > 0xDC00) [[unlikely]]
+	    if (__u2 < 0xD800 || __u2 >= 0xDC00) [[unlikely]]
 	      __c = _S_error(); // unpaired low surrogate
 	    else
 	      {
diff --git a/libstdc++-v3/testsuite/ext/unicode/view.cc b/libstdc++-v3/testsuite/ext/unicode/view.cc
index 4ccf646e0945..40c8fcf34fb7 100644
--- a/libstdc++-v3/testsuite/ext/unicode/view.cc
+++ b/libstdc++-v3/testsuite/ext/unicode/view.cc
@@ -94,6 +94,11 @@ test_illformed_utf16()
   compare(uc::_Utf16_view(s4), u"\uFFFD\uFFFD"sv);
   std::array s5{ s[1], s[0], s[1] };
   compare(uc::_Utf16_view(s5), u"\uFFFD\N{CLOWN FACE}"sv);
+
+  std::array<char16_t, 2> s6{ 0xDC00, 0xDC01 };
+  compare(uc::_Utf16_view(s6), u"\uFFFD\uFFFD"sv);
+  std::array<char16_t, 2> s7{ 0xD7FF, 0xDC00 };
+  compare(uc::_Utf16_view(s7), u"\uD7FF\uFFFD"sv);
 }
 
 constexpr void
-- 
2.51.0