1 files changed, 25 insertions, 0 deletions
diff --git a/src/test/ui/nll/relate_tys/var-appears-twice.rs b/src/test/ui/nll/relate_tys/var-appears-twice.rs
new file mode 100644
index 000000000..77129f446
--- /dev/null
+++ b/src/test/ui/nll/relate_tys/var-appears-twice.rs
@@ -0,0 +1,25 @@
+// Test that the NLL `relate_tys` code correctly deduces that a
+// function returning always its first argument can be upcast to one
+// that returns either first or second argument.
+
+use std::cell::Cell;
+
+type DoubleCell<A> = Cell<(A, A)>;
+type DoublePair<A> = (A, A);
+
+fn make_cell<'b>(x: &'b u32) -> Cell<(&'static u32, &'b u32)> {
+    panic!()
+}
+
+fn main() {
+    let a: &'static u32 = &22;
+    let b = 44;
+
+    // Here we get an error because `DoubleCell<_>` requires the same type
+    // on both parts of the `Cell`, and we can't have that.
+    let x: DoubleCell<_> = make_cell(&b); //~ ERROR
+
+    // Here we do not get an error because `DoublePair<_>` permits
+    // variance on the lifetimes involved.
+    let y: DoublePair<_> = make_cell(&b).get();
+}