Double check that hidden types match the expected hidden type

tests/ui/type-alias-impl-trait/hidden_type_mismatch.rs

@@ -0,0 +1,57 @@
+//! This test checks that we don't lose hidden types
+//! for *other* opaque types that we register and use
+//! to prove bounds while checking that a hidden type
+//! satisfies its opaque type's bounds.
+
+#![feature(trivial_bounds, type_alias_impl_trait)]
+#![allow(trivial_bounds)]
+
+mod sus {
+    use super::*;
+    pub type Sep = impl Sized + std::fmt::Display;
+    //~^ ERROR: concrete type differs from previous defining opaque type use
+    pub fn mk_sep() -> Sep {
+        String::from("hello")
+    }
+
+    pub trait Proj {
+        type Assoc;
+    }
+    impl Proj for () {
+        type Assoc = sus::Sep;
+    }
+
+    pub struct Bar<T: Proj> {
+        pub inner: <T as Proj>::Assoc,
+        pub _marker: T,
+    }
+    impl<T: Proj> Clone for Bar<T> {
+        fn clone(&self) -> Self {
+            todo!()
+        }
+    }
+    impl<T: Proj<Assoc = i32> + Copy> Copy for Bar<T> {}
+    // This allows producing `Tait`s via `From`, even though
+    // `define_tait` is not actually callable, and thus assumed
+    // `Bar<()>: Copy` even though it isn't.
+    pub type Tait = impl Copy + From<Bar<()>> + Into<Bar<()>>;
+    pub fn define_tait() -> Tait
+    where
+        // this proves `Bar<()>: Copy`, but `define_tait` is
+        // now uncallable
+        (): Proj<Assoc = i32>,
+    {
+        Bar { inner: 1i32, _marker: () }
+    }
+}
+
+fn copy_tait(x: sus::Tait) -> (sus::Tait, sus::Tait) {
+    (x, x)
+}
+
+fn main() {
+    let bar = sus::Bar { inner: sus::mk_sep(), _marker: () };
+    let (y, z) = copy_tait(bar.into()); // copy a string
+    drop(y.into()); // drop one instance
+    println!("{}", z.into().inner); // print the other
+}

tests/ui/type-alias-impl-trait/hidden_type_mismatch.stderr

@@ -0,0 +1,14 @@
+error: concrete type differs from previous defining opaque type use
+  --> $DIR/hidden_type_mismatch.rs:11:20
+   |
+LL |     pub type Sep = impl Sized + std::fmt::Display;
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `i32`, got `String`
+   |
+note: previous use here
+  --> $DIR/hidden_type_mismatch.rs:37:21
+   |
+LL |     pub type Tait = impl Copy + From<Bar<()>> + Into<Bar<()>>;
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

tests/ui/type-alias-impl-trait/nested-rpit-with-lifetimes.rs

@@ -1,7 +1,7 @@
-// check-pass
-
 // Regression test for issue #83190, triggering an ICE in borrowck.
 
+// check-pass
+
 pub trait Any {}
 impl<T> Any for T {}