Don't ICE when evaluating writes to uninhabited enum variants

src/test/ui/consts/const-eval/write-to-uninhabited-enum-variant.rs

@@ -0,0 +1,28 @@
+// run-pass
+
+#![allow(dead_code)]
+
+enum Empty { }
+enum Test1 {
+    A(u8),
+    B(Empty),
+}
+enum Test2 {
+    A(u8),
+    B(Empty),
+    C,
+}
+
+fn bar() -> Option<Empty> {
+    None
+}
+
+fn main() {
+    if let Some(x) = bar() {
+        Test1::B(x);
+    }
+
+    if let Some(x) = bar() {
+        Test2::B(x);
+    }
+}