2 files changed, 16 insertions, 11 deletions
diff --git a/dhall/src/semantics/core/var.rs b/dhall/src/semantics/core/var.rs
index 28ddadd..98b315e 100644
--- a/dhall/src/semantics/core/var.rs
+++ b/dhall/src/semantics/core/var.rs
@@ -9,6 +9,8 @@ use crate::syntax::{ExprKind, InterpolatedTextContents, Label, V};
 pub struct AlphaVar {
     normal: V<Label>,
     alpha: V<()>,
+    /// Id of the corresponding binder.
+    binder_uid: BinderUID,
 }
 
 type BinderUID = u64;
@@ -93,6 +95,7 @@ impl Shift for AlphaVar {
         Some(AlphaVar {
             normal: self.normal.shift(delta, &var.normal)?,
             alpha: self.alpha.shift(delta, &var.alpha)?,
+            binder_uid: self.binder_uid,
         })
     }
 }
@@ -121,23 +124,24 @@ impl std::fmt::Debug for Binder {
     }
 }
 
-impl From<Label> for AlphaVar {
-    fn from(x: Label) -> AlphaVar {
+impl<'a> From<&'a Label> for AlphaVar {
+    fn from(x: &'a Label) -> AlphaVar {
         AlphaVar {
-            normal: V(x, 0),
+            normal: V(x.clone(), 0),
             alpha: V((), 0),
+            // TODO: evil evil but only used in shift
+            binder_uid: 0,
         }
     }
 }
-impl<'a> From<&'a Label> for AlphaVar {
-    fn from(x: &'a Label) -> AlphaVar {
-        x.clone().into()
-    }
-}
 impl From<Binder> for AlphaVar {
     fn from(x: Binder) -> AlphaVar {
-        let l: Label = x.into();
-        l.into()
+        let binder_uid = x.uid;
+        AlphaVar {
+            normal: V(x.into(), 0),
+            alpha: V((), 0),
+            binder_uid,
+        }
     }
 }
 impl<'a> From<&'a Binder> for AlphaVar {
diff --git a/dhall/src/semantics/phase/typecheck.rs b/dhall/src/semantics/phase/typecheck.rs
index 926598d..7e408eb 100644
--- a/dhall/src/semantics/phase/typecheck.rs
+++ b/dhall/src/semantics/phase/typecheck.rs
@@ -494,10 +494,11 @@ fn type_last_layer(
                         ValueKind::UnionType(kts) => match kts.get(&x) {
                             // Constructor has type T -> < x: T, ... >
                             Some(Some(t)) => {
+                                let x = ctx.new_binder(x);
                                 RetTypeOnly(
                                     tck_pi_type(
                                         ctx,
-                                        ctx.new_binder(x),
+                                        x.clone(),
                                         t.clone(),
                                         r.under_binder(x),
                                     )?