C++: I think this is more correct.

geoffw0 · geoffw0 · commit a655124213f9 · 2020-08-11T17:28:31.000+01:00
diff --git a/cpp/ql/src/semmle/code/cpp/models/implementations/StdString.qll b/cpp/ql/src/semmle/code/cpp/models/implementations/StdString.qll
@@ -36,7 +36,7 @@ class StdStringPlus extends TaintFunction {
       input.isParameterDeref(0) or
       input.isParameterDeref(1)
     ) and
-    output.isReturnValueDeref()
+    output.isReturnValue()
   }
 }
 

Original file line number	Diff line number	Diff line change
`@@ -36,7 +36,7 @@ class StdStringPlus extends TaintFunction {`
`36`	`36`	`input.isParameterDeref(0) or`
`37`	`37`	`input.isParameterDeref(1)`
`38`	`38`	`) and`
`39`		`- output.isReturnValueDeref()`
	`39`	`+ output.isReturnValue()`
`40`	`40`	`}`
`41`	`41`	`}`
`42`	`42`