[committed] libstdc++: Fix std::spanstream move assignment [PR104032]

diff --git a/libstdc++-v3/include/std/spanstream b/libstdc++-v3/include/std/spanstream
index 240866ff26f..000bda52a1e 100644
--- a/libstdc++-v3/include/std/spanstream
+++ b/libstdc++-v3/include/std/spanstream
@@ -75,10 +75,17 @@  template<typename _CharT, typename _Traits = char_traits<_CharT>>
 
     basic_spanbuf(const basic_spanbuf&) = delete;
 
-    /// Move constructor. In this implementation `rhs` is left unchanged.
+    /** Move constructor.
+     *
+     * Transfers the buffer and pointers into the get and put areas from
+     * `__rhs` to `*this`.
+     *
+     * In this implementation `rhs` is left unchanged,
+     * but that is not guaranteed by the standard.
+     */
     basic_spanbuf(basic_spanbuf&& __rhs)
-    : __streambuf_type(__rhs), _M_mode(__rhs._M_mode)
-    { span(__rhs._M_buf); }
+    : __streambuf_type(__rhs), _M_mode(__rhs._M_mode), _M_buf(__rhs._M_buf)
+    { }
 
     // [spanbuf.assign], assignment and swap
     basic_spanbuf& operator=(const basic_spanbuf&) = delete;
@@ -86,7 +93,7 @@  template<typename _CharT, typename _Traits = char_traits<_CharT>>
     basic_spanbuf&
     operator=(basic_spanbuf&& __rhs)
     {
-      basic_spanbuf(std::move(__rhs))->swap(*this);
+      basic_spanbuf(std::move(__rhs)).swap(*this);
       return *this;
     }
 
diff --git a/libstdc++-v3/testsuite/27_io/spanstream/2.cc b/libstdc++-v3/testsuite/27_io/spanstream/2.cc
new file mode 100644
index 00000000000..a13a50b0dce
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/spanstream/2.cc
@@ -0,0 +1,113 @@ 
+// { dg-options "-std=gnu++23" }
+// { dg-do run { target c++23 } }
+
+#include <spanstream>
+#include <testsuite_hooks.h>
+
+using std::ispanstream;
+using std::ospanstream;
+using std::span;
+
+void
+test_move()
+{
+  char c;
+  {
+    const char str[] = "chars";
+    std::ispanstream a(str);
+    std::ispanstream b = std::move(a);
+    VERIFY( b.span().data() == str && b.span().size() == 6 );
+    VERIFY( b >> c );
+    VERIFY( c == 'c' );
+
+    a = std::move(b);
+    VERIFY( a.span().data() == str && a.span().size() == 6 );
+    VERIFY( a >> c >> c );
+    VERIFY( c == 'a' );
+  }
+
+  {
+    char buf[10] = {};
+    std::ospanstream a(buf);
+    std::ospanstream b = std::move(a);
+    VERIFY( b << 'c' );
+    VERIFY( buf[0] == 'c' );
+    VERIFY( !std::char_traits<char>::compare(buf, "c", 2) );
+
+    a = std::move(b);
+    VERIFY( a << 'h' << 'a' << "rs" );
+    VERIFY( !std::char_traits<char>::compare(buf, "chars", 6) );
+  }
+
+  {
+    char buf[10] = {};
+    std::spanstream a(buf);
+    std::spanstream b = std::move(a);
+    VERIFY( b.span().empty() );
+    VERIFY( b << 'c' );
+    VERIFY( buf[0] == 'c' );
+    VERIFY( !std::char_traits<char>::compare(buf, "c", 2) );
+    VERIFY( b.span().data() == buf && b.span().size() == 1 );
+    VERIFY( b >> c );
+    VERIFY( c == 'c' );
+
+    a = std::move(b);
+    VERIFY( a.span().data() == buf && a.span().size() == 1 );
+    VERIFY( a << 'h' << 'a' << "rs" );
+    VERIFY( !std::char_traits<char>::compare(buf, "chars", 6) );
+    VERIFY( a.span().data() == buf && a.span().size() == 5 );
+  }
+}
+
+void
+test_swap()
+{
+  {
+    const char str1[] = "chars";
+    const char str2[] = "STRING";
+    std::ispanstream a(str1);
+    std::ispanstream b(str2);
+    a.swap(b);
+    VERIFY( a.span().data() == str2 && a.span().size() == 7 );
+    VERIFY( b.span().data() == str1 && b.span().size() == 6 );
+    char c;
+    VERIFY( a >> c );
+    VERIFY( c == 'S' );
+    VERIFY( b >> c );
+    VERIFY( c == 'c' );
+
+    swap(a, b);
+    VERIFY( a.span().data() == str1 && a.span().size() == 6 );
+    VERIFY( b.span().data() == str2 && b.span().size() == 7 );
+    VERIFY( a >> c >> c );
+    VERIFY( c == 'a' );
+    VERIFY( b >> c >> c );
+    VERIFY( c == 'R' );
+  }
+
+  {
+    char buf1[] = "xxxxxxxxxxxxxxx";
+    char buf2[] = "xxxxxxxxxxxxxxx";
+    std::ospanstream a(buf1);
+    std::ospanstream b(buf2);
+    a.swap(b);
+    VERIFY( a << "STR" );
+    VERIFY( !std::char_traits<char>::compare(buf2, "STRx", 4) );
+    VERIFY( b << 'c' << 'h' );
+    VERIFY( !std::char_traits<char>::compare(buf1, "chx", 3) );
+
+    swap(a, b);
+    VERIFY( a.span().size() == 2 );
+    VERIFY( b.span().size() == 3 );
+    VERIFY( a << 'a' << "rs" );
+    VERIFY( !std::char_traits<char>::compare(buf1, "charsx", 6) );
+    VERIFY( b << "IN" << 'G' );
+    VERIFY( !std::char_traits<char>::compare(buf2, "STRINGx", 7) );
+  }
+}
+
+int main()
+{
+  test_move();
+  test_swap();
+}