1 files changed, 56 insertions, 0 deletions
diff --git a/src/test/ui/hrtb/hrtb-perfect-forwarding.rs b/src/test/ui/hrtb/hrtb-perfect-forwarding.rs
new file mode 100644
index 000000000..d45fa183c
--- /dev/null
+++ b/src/test/ui/hrtb/hrtb-perfect-forwarding.rs
@@ -0,0 +1,56 @@
+// Test a case where you have an impl of `Foo<X>` for all `X` that
+// is being applied to `for<'a> Foo<&'a mut X>`. Issue #19730.
+
+trait Foo<X> {
+    fn foo(&mut self, x: X) {}
+}
+
+trait Bar<X> {
+    fn bar(&mut self, x: X) {}
+}
+
+impl<'a, X, F> Foo<X> for &'a mut F where F: Foo<X> + Bar<X> {}
+
+impl<'a, X, F> Bar<X> for &'a mut F where F: Bar<X> {}
+
+fn no_hrtb<'b, T>(mut t: T) //~ WARN function cannot return
+where
+    T: Bar<&'b isize>,
+{
+    // OK -- `T : Bar<&'b isize>`, and thus the impl above ensures that
+    // `&mut T : Bar<&'b isize>`.
+    no_hrtb(&mut t);
+}
+
+fn bar_hrtb<T>(mut t: T) //~ WARN function cannot return
+where
+    T: for<'b> Bar<&'b isize>,
+{
+    // OK -- `T : for<'b> Bar<&'b isize>`, and thus the impl above
+    // ensures that `&mut T : for<'b> Bar<&'b isize>`.  This is an
+    // example of a "perfect forwarding" impl.
+    bar_hrtb(&mut t);
+}
+
+fn foo_hrtb_bar_not<'b, T>(mut t: T) //~ WARN function cannot return
+where
+    T: for<'a> Foo<&'a isize> + Bar<&'b isize>,
+{
+    // Not OK -- The forwarding impl for `Foo` requires that `Bar` also
+    // be implemented. Thus to satisfy `&mut T : for<'a> Foo<&'a
+    // isize>`, we require `T : for<'a> Bar<&'a isize>`, but the where
+    // clause only specifies `T : Bar<&'b isize>`.
+    foo_hrtb_bar_not(&mut t);
+    //~^ ERROR implementation of `Bar` is not general enough
+    //~^^ ERROR lifetime may not live long enough
+}
+
+fn foo_hrtb_bar_hrtb<T>(mut t: T) //~ WARN function cannot return
+where
+    T: for<'a> Foo<&'a isize> + for<'b> Bar<&'b isize>,
+{
+    // OK -- now we have `T : for<'b> Bar<&'b isize>`.
+    foo_hrtb_bar_hrtb(&mut t);
+}
+
+fn main() {}