Ensure that we never try to monomorphize the upcasting of impossible …

…dyn types

compiler/rustc_trait_selection/src/traits/vtable.rs

@@ -234,6 +234,11 @@ fn vtable_entries<'tcx>(
         "vtable trait ref should be normalized"
     );
 
+    // We're monomorphizing a dyn trait object that can never be constructed.
+    if tcx.instantiate_and_check_impossible_predicates((trait_ref.def_id, trait_ref.args)) {
+        return TyCtxt::COMMON_VTABLE_ENTRIES;
+    }
+
     debug!("vtable_entries({:?})", trait_ref);
 
     let mut entries = vec![];
@@ -323,6 +328,14 @@ pub(crate) fn first_method_vtable_slot<'tcx>(tcx: TyCtxt<'tcx>, key: ty::TraitRe
         source.principal().unwrap().with_self_ty(tcx, tcx.types.trait_object_dummy_self),
     );
 
+    // We're monomorphizing a dyn trait object that can never be constructed.
+    if tcx.instantiate_and_check_impossible_predicates((
+        source_principal.def_id,
+        source_principal.args,
+    )) {
+        return 0;
+    }
+
     let target_principal = ty::ExistentialTraitRef::erase_self_ty(tcx, key);
 
     let vtable_segment_callback = {
@@ -388,6 +401,14 @@ pub(crate) fn supertrait_vtable_slot<'tcx>(
         source.principal().unwrap().with_self_ty(tcx, tcx.types.trait_object_dummy_self),
     );
 
+    // We're monomorphizing a dyn trait object that can never be constructed.
+    if tcx.instantiate_and_check_impossible_predicates((
+        source_principal.def_id,
+        source_principal.args,
+    )) {
+        return None;
+    }
+
     let vtable_segment_callback = {
         let mut vptr_offset = 0;
         move |segment| {

tests/ui/traits/trait-upcasting/mono-impossible.rs

@@ -0,0 +1,24 @@
+#![feature(trait_upcasting)]
+
+trait Supertrait<T> {
+    fn method(&self) {}
+}
+impl<T> Supertrait<T> for () {}
+
+trait WithAssoc {
+    type Assoc;
+}
+trait Trait<P: WithAssoc>: Supertrait<P::Assoc> + Supertrait<()> {}
+
+fn upcast<P>(x: &dyn Trait<P>) -> &dyn Supertrait<()> {
+    x
+}
+
+fn call<P>(x: &dyn Trait<P>) {
+    x.method();
+}
+
+fn main() {
+    println!("{:p}", upcast::<()> as *const ());
+    println!("{:p}", call::<()> as *const ());
+}