Add regression test for CoerceShared impl with unnormalizable target

tests/ui/reborrow/coerce-shared-unnormalizable-target-issue-156699.rs

@@ -0,0 +1,17 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/156699>.
+//!
+//! The target type of a `CoerceShared` impl that could not be normalized
+//! used to cause an ICE instead of an error.
+
+#![feature(reborrow)]
+
+use std::marker::{CoerceShared, Reborrow};
+
+struct Wrapper<'a, T>(&'a mut T);
+
+impl<'a, T> Reborrow for Wrapper<'a, T> {}
+
+impl<'a, T> CoerceShared<<T as Iterator>::Item> for Wrapper<'a, T> {}
+//~^ ERROR `T` is not an iterator
+
+fn main() {}

tests/ui/reborrow/coerce-shared-unnormalizable-target-issue-156699.stderr

@@ -0,0 +1,14 @@
+error[E0277]: `T` is not an iterator
+  --> $DIR/coerce-shared-unnormalizable-target-issue-156699.rs:14:1
+   |
+LL | impl<'a, T> CoerceShared<<T as Iterator>::Item> for Wrapper<'a, T> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `T` is not an iterator
+   |
+help: consider restricting type parameter `T` with trait `Iterator`
+   |
+LL | impl<'a, T: std::iter::Iterator> CoerceShared<<T as Iterator>::Item> for Wrapper<'a, T> {}
+   |           +++++++++++++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.