fix overlapping memcpy in string::assign

include/boost/json/impl/string.ipp

@@ -176,9 +176,18 @@ assign(
     char const* s,
     size_type count)
 {
-    std::char_traits<char>::copy(
-        impl_.assign(count, sp_),
-        s, count);
+    auto const p = impl_.data();
+    if(detail::ptr_in_range(p, p + impl_.size(), s))
+    {
+        std::char_traits<char>::move(p, s, count);
+        impl_.term(count);
+    }
+    else
+    {
+        std::char_traits<char>::copy(
+            impl_.assign(count, sp_),
+            s, count);
+    }
     return *this;
 }
 

test/string.cpp

@@ -840,6 +840,36 @@ class string_test
             });
         };
 
+        // assign(char const*, size_type) self-referencing
+        {
+            // non-SBO: assign from a substring of itself
+            {
+                string s("abcdefghijklmnopqrstuvwxyz0123456789");
+                string_view sub(s.data() + 3, s.size() - 3);
+                std::string expected(sub.data(), sub.size());
+                s.assign(sub);
+                BOOST_TEST(s == expected);
+            }
+
+            // non-SBO: assign from beginning (prefix)
+            {
+                string s("abcdefghijklmnopqrstuvwxyz0123456789");
+                string_view sub(s.data(), 10);
+                std::string expected(sub.data(), sub.size());
+                s.assign(sub);
+                BOOST_TEST(s == expected);
+            }
+
+            // non-SBO: self-assign via string_view
+            {
+                string s("abcdefghijklmnopqrstuvwxyz0123456789");
+                string_view sub(s.data(), s.size());
+                std::string expected(sub.data(), sub.size());
+                s.assign(sub);
+                BOOST_TEST(s == expected);
+            }
+        }
+
         // assign(char const* s)
         {
             fail_loop([&](storage_ptr const& sp)