add non-regression test for issue 126670

tests/ui/impl-trait/in-trait/refine-resolution-errors.rs

@@ -0,0 +1,25 @@
+// This is a non-regression test for issue #126670 where RPITIT refinement checking encountered
+// errors during resolution and ICEd.
+
+//@ edition: 2018
+
+pub trait A {}
+
+pub trait Mirror {
+    type Assoc: ?Sized;
+}
+impl<T: ?Sized> Mirror for dyn A {
+    //~^ ERROR the type parameter `T` is not constrained
+    type Assoc = T;
+}
+
+pub trait First {
+    async fn first() -> <dyn A + 'static as Mirror>::Assoc;
+    //~^ ERROR type annotations needed
+}
+
+impl First for dyn A {
+    async fn first() {}
+}
+
+fn main() {}

tests/ui/impl-trait/in-trait/refine-resolution-errors.stderr

@@ -0,0 +1,16 @@
+error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/refine-resolution-errors.rs:11:6
+   |
+LL | impl<T: ?Sized> Mirror for dyn A {
+   |      ^ unconstrained type parameter
+
+error[E0282]: type annotations needed
+  --> $DIR/refine-resolution-errors.rs:17:5
+   |
+LL |     async fn first() -> <dyn A + 'static as Mirror>::Assoc;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0207, E0282.
+For more information about an error, try `rustc --explain E0207`.