diff --git a/src/librustc_mir/interpret/validity.rs b/src/librustc_mir/interpret/validity.rs
index 84aa5b67566..38170949734 100644
--- a/src/librustc_mir/interpret/validity.rs
+++ b/src/librustc_mir/interpret/validity.rs
@@ -13,7 +13,7 @@
 use std::ops::RangeInclusive;
 
 use syntax_pos::symbol::Symbol;
-use rustc::ty::layout::{self, Size, Align, TyLayout, LayoutOf};
+use rustc::ty::layout::{self, Size, Align, TyLayout, LayoutOf, VariantIdx};
 use rustc::ty;
 use rustc_data_structures::fx::FxHashSet;
 use rustc::mir::interpret::{
@@ -74,6 +74,7 @@ macro_rules! try_validation {
 #[derive(Copy, Clone, Debug)]
 pub enum PathElem {
     Field(Symbol),
+    Variant(Symbol),
     ClosureVar(Symbol),
     ArrayElem(usize),
     TupleElem(usize),
@@ -107,6 +108,7 @@ fn path_format(path: &Vec<PathElem>) -> String {
     for elem in path.iter() {
         match elem {
             Field(name) => write!(out, ".{}", name),
+            Variant(name) => write!(out, ".<downcast-variant({})>", name),
             ClosureVar(name) => write!(out, ".<closure-var({})>", name),
             TupleElem(idx) => write!(out, ".{}", idx),
             ArrayElem(idx) => write!(out, "[{}]", idx),
@@ -192,9 +194,11 @@ fn push_aggregate_field_path_elem(
                     layout::Variants::Single { index } =>
                         // Inside a variant
                         PathElem::Field(def.variants[index].fields[field].ident.name),
-                    _ =>
-                        // To a variant
-                        PathElem::Field(def.variants[field].name)
+                    _ => {
+                        // Enums have no fields other than their tag
+                        assert_eq!(field, 0);
+                        PathElem::Tag
+                    }
                 }
             }
 
@@ -241,6 +245,24 @@ fn visit_field(
         Ok(())
     }
 
+    #[inline]
+    fn visit_variant(
+        &mut self,
+        old_op: OpTy<'tcx, M::PointerTag>,
+        variant_id: VariantIdx,
+        new_op: OpTy<'tcx, M::PointerTag>
+    ) -> EvalResult<'tcx> {
+        // Remember the old state
+        let path_len = self.path.len();
+        // Perform operation
+        let name = old_op.layout.ty.ty_adt_def().unwrap().variants[variant_id].name;
+        self.path.push(PathElem::Variant(name));
+        self.visit_value(new_op)?;
+        // Undo changes
+        self.path.truncate(path_len);
+        Ok(())
+    }
+
     #[inline]
     fn visit_value(&mut self, op: OpTy<'tcx, M::PointerTag>) -> EvalResult<'tcx>
     {
diff --git a/src/librustc_mir/interpret/visitor.rs b/src/librustc_mir/interpret/visitor.rs
index 392e2791809..4a470456432 100644
--- a/src/librustc_mir/interpret/visitor.rs
+++ b/src/librustc_mir/interpret/visitor.rs
@@ -1,7 +1,7 @@
 //! Visitor for a run-time value with a given layout: Traverse enums, structs and other compound
 //! types until we arrive at the leaves, with custom handling for primitive types.
 
-use rustc::ty::layout::{self, TyLayout};
+use rustc::ty::layout::{self, TyLayout, VariantIdx};
 use rustc::ty;
 use rustc::mir::interpret::{
     EvalResult,
@@ -32,7 +32,7 @@ fn to_op(
     fn project_downcast(
         self,
         ecx: &EvalContext<'a, 'mir, 'tcx, M>,
-        variant: usize,
+        variant: VariantIdx,
     ) -> EvalResult<'tcx, Self>;
 
     /// Project to the n-th field.
@@ -70,7 +70,7 @@ fn from_mem_place(mplace: MPlaceTy<'tcx, M::PointerTag>) -> Self {
     fn project_downcast(
         self,
         ecx: &EvalContext<'a, 'mir, 'tcx, M>,
-        variant: usize,
+        variant: VariantIdx,
     ) -> EvalResult<'tcx, Self> {
         ecx.operand_downcast(self, variant)
     }
@@ -109,7 +109,7 @@ fn from_mem_place(mplace: MPlaceTy<'tcx, M::PointerTag>) -> Self {
     fn project_downcast(
         self,
         ecx: &EvalContext<'a, 'mir, 'tcx, M>,
-        variant: usize,
+        variant: VariantIdx,
     ) -> EvalResult<'tcx, Self> {
         ecx.mplace_downcast(self, variant)
     }
@@ -171,6 +171,16 @@ fn visit_field(
                 self.visit_value(new_val)
             }
 
+            #[inline(always)]
+            fn visit_variant(
+                &mut self,
+                _old_val: Self::V,
+                _variant: VariantIdx,
+                new_val: Self::V,
+            ) -> EvalResult<'tcx> {
+                self.visit_value(new_val)
+            }
+
             /// Called whenever we reach a value with uninhabited layout.
             /// Recursing to fields will *always* continue after this!  This is not meant to control
             /// whether and how we descend recursively/ into the scalar's fields if there are any,
@@ -221,7 +231,7 @@ fn walk_value(&mut self, v: Self::V) -> EvalResult<'tcx>
                         let inner = v.project_downcast(self.ecx(), idx)?;
                         trace!("walk_value: variant layout: {:#?}", inner.layout());
                         // recurse with the inner type
-                        return self.visit_field(v, idx, inner);
+                        return self.visit_variant(v, idx, inner);
                     }
                     layout::Variants::Single { .. } => {}
                 }
diff --git a/src/test/ui/consts/const-eval/ub-enum.stderr b/src/test/ui/consts/const-eval/ub-enum.stderr
index 804c9643d8a..d36e96b0817 100644
--- a/src/test/ui/consts/const-eval/ub-enum.stderr
+++ b/src/test/ui/consts/const-eval/ub-enum.stderr
@@ -42,7 +42,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-enum.rs:61:1
    |
 LL | const BAD_ENUM_CHAR: Option<(char, char)> = Some(('x', unsafe { TransmuteChar { a: !0 }.b }));
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 4294967295 at .Some.0.1, but expected something less or equal to 1114111
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 4294967295 at .<downcast-variant(Some)>.0.1, but expected something less or equal to 1114111
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior