1 files changed, 22 insertions, 0 deletions
diff --git a/compiler/rustc_typeck/src/variance/xform.rs b/compiler/rustc_typeck/src/variance/xform.rs
new file mode 100644
index 000000000..027f0859f
--- /dev/null
+++ b/compiler/rustc_typeck/src/variance/xform.rs
@@ -0,0 +1,22 @@
+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,
+    }
+}