Switch upcast projections to allowing opaque types and add a test showing it works.

The old solver was already ICEing on this test before this change

Author: oli-obk

tests/ui/traits/trait-upcasting/illegal-upcast-from-impl-opaque.current.stderr

@@ -0,0 +1,17 @@
+error[E0308]: mismatched types
+  --> $DIR/illegal-upcast-from-impl-opaque.rs:26:5
+   |
+LL | type Foo = impl Sized;
+   |            ---------- the found opaque type
+LL |
+LL | fn illegal(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
+   |                                         ----------------------- expected `&dyn Super<Assoc = i32>` because of return type
+LL |     x
+   |     ^ expected trait `Super`, found trait `Sub`
+   |
+   = note: expected reference `&dyn Super<Assoc = i32>`
+              found reference `&dyn Sub<Assoc = Foo>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/traits/trait-upcasting/illegal-upcast-from-impl-opaque.next.stderr

@@ -0,0 +1,14 @@
+error: internal compiler error: error performing operation: query type op
+  --> $DIR/illegal-upcast-from-impl-opaque.rs:25:1
+   |
+LL | fn illegal(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: 
+  --> $DIR/illegal-upcast-from-impl-opaque.rs:25:1
+   |
+LL | fn illegal(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+query stack during panic:
+end of query stack

tests/ui/traits/trait-upcasting/illegal-upcast-from-impl-opaque.rs

@@ -0,0 +1,29 @@
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@[next] failure-status: 101
+//@[next] known-bug: unknown
+//@[next] normalize-stderr-test "note: .*\n\n" -> ""
+//@[next] normalize-stderr-test "thread 'rustc' panicked.*\n.*\n" -> ""
+//@[next] normalize-stderr-test "(error: internal compiler error: [^:]+):\d+:\d+: " -> "$1:LL:CC: "
+//@[next] normalize-stderr-test "delayed at .*" -> ""
+//@[next] rustc-env:RUST_BACKTRACE=0
+
+#![feature(trait_upcasting, type_alias_impl_trait)]
+
+trait Super {
+    type Assoc;
+}
+
+trait Sub: Super {}
+
+impl<T: ?Sized> Super for T {
+    type Assoc = i32;
+}
+
+type Foo = impl Sized;
+
+fn illegal(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
+    x //[current]~ mismatched types
+}
+
+fn main() {}