Abstract
We show how the subtype relation of the well-known system Fsub, the
second-order polymorphic lambda-calculus with bounded universal type
quantification and subtyping, due to Cardelli, Wegner, Bruce, Longo, Curien,
Ghelli, proved undecidable by Pierce (POPL'92), can be interpreted in the
(weak) monadic second-order theory of one (B\"uchi), two (Rabin), several, or
infinitely many successor functions. These (W)SnS-interpretations show that the
undecidable system Fsub possesses consistent decidable extensions, i.e., Fsub
is not essentially undecidable (Tarski, 1949). \par We demonstrate an infinite
class of structural decidable extensions of Fsub, which combine traditional
subtype inference rules with the above (W)SnS-interpretations. All these
extensions, which we call systems FsubSnS, are still more powerful than Fsub,
but less coarse than the direct (W)SnS-interpretations. \par The main
distinctive features of the systems FsubSnS are: 1) decidability, 2) closure
w.r.t.\ transitivity; 3)
structuredness, e.g., they never subtype a functional type to a universal one
or vice versa, 4) they all contain the powerful rule for subtyping boundedly
quantified types.
