Don't drop Upcast candidate in intercrate mode

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

@@ -921,6 +921,12 @@ fn need_migrate_deref_output_trait_object(
         param_env: ty::ParamEnv<'tcx>,
         cause: &ObligationCause<'tcx>,
     ) -> Option<ty::PolyExistentialTraitRef<'tcx>> {
+        // Don't drop any candidates in intercrate mode, as it's incomplete.
+        // (Not that it matters, since `Unsize` is not a stable trait.)
+        if self.infcx.intercrate {
+            return None;
+        }
+
         let tcx = self.tcx();
         if tcx.features().trait_upcasting {
             return None;

tests/ui/specialization/dont-drop-upcast-candidate.rs

@@ -0,0 +1,13 @@
+#![feature(unsize)]
+
+use std::marker::Unsize;
+use std::ops::Deref;
+
+trait Foo: Bar {}
+trait Bar {}
+
+impl<T> Bar for T where dyn Foo: Unsize<dyn Bar> {}
+impl Bar for () {}
+//~^ ERROR conflicting implementations of trait `Bar` for type `()`
+
+fn main() {}

tests/ui/specialization/dont-drop-upcast-candidate.stderr

@@ -0,0 +1,11 @@
+error[E0119]: conflicting implementations of trait `Bar` for type `()`
+  --> $DIR/dont-drop-upcast-candidate.rs:10:1
+   |
+LL | impl<T> Bar for T where dyn Foo: Unsize<dyn Bar> {}
+   | ------------------------------------------------ first implementation here
+LL | impl Bar for () {}
+   | ^^^^^^^^^^^^^^^ conflicting implementation for `()`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.