Avoid emitting a unhelpful cascading resolution error.

Author: jseyfried

src/librustc_resolve/lib.rs

@@ -1810,6 +1810,8 @@ impl<'a> Resolver<'a> {
             if let Def::Trait(_) = path_res.base_def {
                 debug!("(resolving trait) found trait def: {:?}", path_res);
                 Ok(path_res)
+            } else if path_res.base_def == Def::Err {
+                Err(true)
             } else {
                 let mut err =
                     resolve_struct_error(self,

src/test/compile-fail/issue-5035.rs

@@ -13,4 +13,8 @@ type K = I;
 //~^ NOTE: aliases cannot be used for traits
 impl K for isize {} //~ ERROR: `K` is not a trait
 //~| is not a trait
+
+use ImportError; //~ ERROR unresolved
+impl ImportError for () {} // check that this is not an additional error (c.f. #35142)
+
 fn main() {}