don't ICE for alias-relate goals with error term

compiler/rustc_next_trait_solver/src/solve/alias_relate.rs

@@ -34,7 +34,17 @@ where
     ) -> QueryResult<I> {
         let cx = self.cx();
         let Goal { param_env, predicate: (lhs, rhs, direction) } = goal;
-        debug_assert!(lhs.to_alias_term().is_some() || rhs.to_alias_term().is_some());
+
+        // Check that the alias-relate goal is reasonable. Writeback for
+        // `coroutine_stalled_predicates` can replace alias terms with
+        // `{type error}` if the alias still contains infer vars, so we also
+        // accept alias-relate goals where one of the terms is an error.
+        debug_assert!(
+            lhs.to_alias_term().is_some()
+                || rhs.to_alias_term().is_some()
+                || lhs.is_error()
+                || rhs.is_error()
+        );
 
         // Structurally normalize the lhs.
         let lhs = if let Some(alias) = lhs.to_alias_term() {

compiler/rustc_type_ir/src/inherent.rs

@@ -126,6 +126,10 @@ pub trait Ty<I: Interner<Ty = Self>>:
         matches!(self.kind(), ty::Infer(ty::TyVar(_)))
     }
 
+    fn is_ty_error(self) -> bool {
+        matches!(self.kind(), ty::Error(_))
+    }
+
     fn is_floating_point(self) -> bool {
         matches!(self.kind(), ty::Float(_) | ty::Infer(ty::FloatVar(_)))
     }
@@ -284,6 +288,10 @@ pub trait Const<I: Interner<Const = Self>>:
     fn is_ct_var(self) -> bool {
         matches!(self.kind(), ty::ConstKind::Infer(ty::InferConst::Var(_)))
     }
+
+    fn is_ct_error(self) -> bool {
+        matches!(self.kind(), ty::ConstKind::Error(_))
+    }
 }
 
 pub trait ValueConst<I: Interner<ValueConst = Self>>: Copy + Debug + Hash + Eq {
@@ -370,6 +378,13 @@ pub trait Term<I: Interner<Term = Self>>:
         }
     }
 
+    fn is_error(self) -> bool {
+        match self.kind() {
+            ty::TermKind::Ty(ty) => ty.is_ty_error(),
+            ty::TermKind::Const(ct) => ct.is_ct_error(),
+        }
+    }
+
     fn to_alias_term(self) -> Option<ty::AliasTerm<I>> {
         match self.kind() {
             ty::TermKind::Ty(ty) => match ty.kind() {