Fixes #6837 (#7271)

tests/cpp/temitlist.nim

@@ -1,6 +1,7 @@
 discard """
   targets: "cpp"
-  output: '''6.0'''
+  output: '''6.0
+0'''
 """
 
 # bug #4730
@@ -20,3 +21,16 @@ proc main =
   echo v[0]
 
 main()
+
+#------------
+
+#bug #6837
+type StdString {.importCpp: "std::string", header: "<string>", byref.} = object
+proc initString(): StdString {.constructor, importCpp: "std::string(@)", header: "<string>".}
+proc size(this: var StdString): csize {.importCpp: "size", header: "<string>".}
+
+proc f(): csize =
+  var myString: StdString = initString()
+  return myString.size()
+
+echo f()