Allow constraining opaque types during auto trait casting

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -1203,7 +1203,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let InferOk { mut obligations, .. } = self
                     .infcx
                     .at(&obligation.cause, obligation.param_env)
-                    .sup(DefineOpaqueTypes::No, target, source_trait)
+                    .sup(DefineOpaqueTypes::Yes, target, source_trait)
                     .map_err(|_| Unimplemented)?;
 
                 // Register one obligation for 'a: 'b.

tests/ui/impl-trait/trait_upcasting.rs

@@ -1,5 +1,7 @@
 //! Test that we allow unsizing `Trait<Concrete>` to `Trait<Opaque>` and vice versa
 
+//@ check-pass
+
 trait Trait<T> {}
 
 impl<T, U> Trait<T> for U {}
@@ -8,7 +10,6 @@ fn hello() -> &'static (dyn Trait<impl Sized> + Send) {
     if false {
         let x = hello();
         let _: &'static dyn Trait<()> = x;
-        //~^ ERROR: mismatched types
     }
     todo!()
 }
@@ -18,7 +19,6 @@ fn bye() -> &'static dyn Trait<impl Sized> {
         let mut x = bye();
         let y: &'static (dyn Trait<()> + Send) = &();
         x = y;
-        //~^ ERROR: mismatched types
     }
     todo!()
 }

tests/ui/impl-trait/trait_upcasting_reference_mismatch.rs

@@ -0,0 +1,18 @@
+//! Show an uninformative diagnostic that we could possibly improve in the future
+
+trait Trait<T> {}
+
+impl<T, U> Trait<T> for U {}
+
+fn hello() -> &'static (dyn Trait<impl Sized> + Send) {
+    //~^ ERROR: type annotations needed
+    if false {
+        let x = hello();
+        let _: &'static dyn Trait<()> = &x;
+        //^ Note the extra `&`, paired with the blanket impl causing
+        // `impl Sized` to never get a hidden type registered.
+    }
+    todo!()
+}
+
+fn main() {}

tests/ui/impl-trait/trait_upcasting_reference_mismatch.stderr

@@ -0,0 +1,9 @@
+error[E0282]: type annotations needed
+  --> $DIR/trait_upcasting_reference_mismatch.rs:7:35
+   |
+LL | fn hello() -> &'static (dyn Trait<impl Sized> + Send) {
+   |                                   ^^^^^^^^^^ cannot infer type
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0282`.