Tuple.toString does not work with mutable toString

[dlangBugzillaToGithub]

timon.gehr reported this on 2019-07-06T17:44:21Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=20031

Description

DMD64 D Compiler v2.086.0:

import std.stdio, std.typecons;
struct S{
    string toString(){ return "not broken"; }
}
void main(){
    writeln(S()); // "not broken"
    writeln(tuple(S(),S())); // "Tuple!(S, S)(const(S)(), const(S)())"
}

The second writeln should print: "Tuple!(S, S)(not broken, not broken)"