Strange XSD.String error

Summary

Originally reported in the forum: https://www.eclipse.org/forums/index.php/t/1116105/

We found a strange compiler error when mixing XSD.String and charstrings in assignments.

function f_Test() {
    var XSD.String v_Str1 := "123";
    var charstring v_Str2 := "456";
    var charstring v_Str3 := v_Str1 & v_Str2;  // <--- ??? causes an error
    var charstring v_Str4 := v_Str2 & v_Str1;  // does not cause an error !
    log(v_Str3);
    log(v_Str4);
}