Nigel Foster. EU Law Directions 6 ed. 2018. p. 215
8.2.2 Indirect effects
Case 8/81 Becker, and its acknowledgement of a wider concept of direct effects as a standard by which the conformity of national law could in effect be reviewed by the CoJ, had already pointed EC (and now EU) law in the direction that was about to be taken in the next development.
Case 14/83 Von Colson provided a solution where national law was not in tune with Community law and direct effects could not provide a remedy. The ruling offers an alternative for individuals defeated by the lack of horizontal direct effects. The case concerned Article 6 of the Equal Treatment Directive 76/207 and a claim against a public employer for lack of adequate compensation when discriminated against.
At the same time, Case 79/83 Harz v Tradex was also heard by the CoJ, which involved a similar claim against a private employer. Rather than highlight the unfortunate results of the lack of horizontal direct effects of Directives against the private, but not the public, employer, the CoJ concentrated on old Article 5 EEC (now 4(3) TEU), which requires member states to comply with Community obligations.
The Court held that this requirement applies to all authorities of member states, including the courts, which are obliged therefore to interpret national law in such a way as to ensure that the obligations of a Directive are obeyed, regardless of whether the national law was based on any particular Directive.
 It would not, of course, have been acceptable for Von Colson to have succeeded under direct effects against an arm of the state but Harz not to have done so against a private employer;  thus, an alternative solution was required. The effectiveness of this remedy, though, depends on the willingness or ability of the member states’ courts to interpret national law, if it exists, to achieve the correct result.
This sentence distinguishes the two: Von Colson involved an arm of the state that can be distinguished from Harx that involved a private employer. Thus what's the quandary?
What was the "alternative solution"?