Entailment of subtype constraints was introduced for constraint simplification in subtype inference systems. Designing an efficient algorithm for subtype entailment turned out to be surprisingly difficult. The situation was clarified by Rehof and Henglein who proved entailment of structural subtype constraints to be coNP-complete for simple types and PSPACE-complete for recursive types. For entailment of non-structural subtype constraints of both simple and recursive types they proved PSPACE-hardness and conjectured PSPACE-completeness but failed in finding a complete algorithm. In this paper, we investigate the source of complications and isolate a natural subproblem of non-structural subtype entailment that we prove PSPACE-complete. We conjecture (but this is left open) that the presented approach can be extended to the general case.
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