[libc++] Fixes istream::sync.

libcxx/include/istream

@@ -1009,17 +1009,18 @@ basic_istream<_CharT, _Traits>& basic_istream<_CharT, _Traits>::unget() {
 template <class _CharT, class _Traits>
 int basic_istream<_CharT, _Traits>::sync() {
   ios_base::iostate __state = ios_base::goodbit;
-  int __r                   = 0;
   sentry __sen(*this, true);
+  if (this->rdbuf() == nullptr)
+    return -1;
+
+  int __r = 0;
   if (__sen) {
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
     try {
 #endif // _LIBCPP_HAS_NO_EXCEPTIONS
-      if (this->rdbuf() == nullptr)
-        return -1;
       if (this->rdbuf()->pubsync() == -1) {
         __state |= ios_base::badbit;
-        return -1;
+        __r = -1;
       }
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
     } catch (...) {

libcxx/test/std/input.output/iostream.format/input.streams/istream.unformatted/sync.pass.cpp

@@ -10,6 +10,12 @@
 
 // int sync();
 
+// The fix for bug 51497 and bug 51499 require and updated dylib due to
+// explicit instantiations. That means Apple backdeployment targets remain
+// broken.
+// TODO(#82107) Enable XFAIL.
+// UNSUPPORTED: using-built-library-before-llvm-19
+
 #include <istream>
 #include <cassert>
 
@@ -48,6 +54,18 @@ struct testbuf
     }
 };
 
+template <class CharT>
+struct testbuf_pubsync_error
+    : public std::basic_streambuf<CharT>
+{
+public:
+
+    testbuf_pubsync_error() {}
+protected:
+    virtual int sync() { return -1; }
+};
+
+
 #ifndef TEST_HAS_NO_EXCEPTIONS
 struct testbuf_exception { };
 
@@ -85,21 +103,62 @@ struct throwing_testbuf
 
 int main(int, char**)
 {
+    {
+        std::istream is(nullptr);
+        assert(is.sync() == -1);
+    }
     {
         testbuf<char> sb(" 123456789");
         std::istream is(&sb);
         assert(is.sync() == 0);
         assert(sync_called == 1);
     }
+    {
+        testbuf_pubsync_error<char> sb;
+        std::istream is(&sb);
+        is.exceptions(std::ios_base::failbit | std::ios_base::eofbit);
+        assert(is.sync() == -1);
+        assert( is.bad());
+        assert(!is.eof());
+        assert( is.fail());
+    }
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
+    {
+        std::wistream is(nullptr);
+        assert(is.sync() == -1);
+    }
     {
         testbuf<wchar_t> sb(L" 123456789");
         std::wistream is(&sb);
         assert(is.sync() == 0);
         assert(sync_called == 2);
     }
+    {
+        testbuf_pubsync_error<wchar_t> sb;
+        std::wistream is(&sb);
+        is.exceptions(std::ios_base::failbit | std::ios_base::eofbit);
+        assert(is.sync() == -1);
+        assert( is.bad());
+        assert(!is.eof());
+        assert( is.fail());
+    }
 #endif
 #ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        testbuf_pubsync_error<char> sb;
+        std::istream is(&sb);
+        is.exceptions(std::ios_base::badbit);
+        bool threw = false;
+        try {
+            is.sync();
+        } catch (std::ios_base::failure const&) {
+            threw = true;
+        }
+        assert( is.bad());
+        assert(!is.eof());
+        assert( is.fail());
+        assert(threw);
+    }
     {
         throwing_testbuf<char> sb(" 123456789");
         std::basic_istream<char> is(&sb);
@@ -116,6 +175,21 @@ int main(int, char**)
         assert(threw);
     }
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
+    {
+        testbuf_pubsync_error<wchar_t> sb;
+        std::wistream is(&sb);
+        is.exceptions(std::ios_base::badbit);
+        bool threw = false;
+        try {
+            is.sync();
+        } catch (std::ios_base::failure const&) {
+            threw = true;
+        }
+        assert( is.bad());
+        assert(!is.eof());
+        assert( is.fail());
+        assert(threw);
+    }
     {
         throwing_testbuf<wchar_t> sb(L" 123456789");
         std::basic_istream<wchar_t> is(&sb);
@@ -131,7 +205,7 @@ int main(int, char**)
         assert( is.fail());
         assert(threw);
     }
-#endif
+#endif // TEST_HAS_NO_WIDE_CHARACTERS
 #endif // TEST_HAS_NO_EXCEPTIONS
 
     return 0;