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use rustc_middle::ty;
pub fn glb(v1: ty::Variance, v2: ty::Variance) -> ty::Variance {
// Greatest lower bound of the variance lattice as
// defined in The Paper:
//
// *
// - +
// o
match (v1, v2) {
(ty::Invariant, _) | (_, ty::Invariant) => ty::Invariant,
(ty::Covariant, ty::Contravariant) => ty::Invariant,
(ty::Contravariant, ty::Covariant) => ty::Invariant,
(ty::Covariant, ty::Covariant) => ty::Covariant,
(ty::Contravariant, ty::Contravariant) => ty::Contravariant,
(x, ty::Bivariant) | (ty::Bivariant, x) => x,
}
}
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