Seiberg and Witten have shown that in N = 2 SQCD with N f = 2N c = 4 the S-duality group PSL(2, Z) acts on the flavor charges, which are weights of Spin (8), by triality. There are other N = 2 SCFTs in which SU(2) SYM is coupled to stronglyinteracting non-Lagrangian matter: their matter charges are weights of E 6 , E 7 and E 8 instead of Spin(8). The S-duality group PSL(2, Z) acts on these weights: what replaces Spin(8) triality for the E 6 , E 7 , E 8 root lattices?In this paper we answer the question. The action on the matter charges of (a finite central extension of) PSL(2, Z) factorizes trough the action of the exceptional ShephardTodd groups G 4 and G 8 which should be seen as complex analogs of the usual triality group S 3 ≃ Weyl(A 2 ). Our analysis is based on the identification of S-duality for SU(2) gauge SCFTs with the group of automorphisms of the cluster category of weighted projective lines of tubular type.