Avoid suggesting constrain the associated type with unknown type

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -1572,26 +1572,31 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                 [ast::PathSegment { args: None, .. }],
                 [ast::GenericBound::Trait(poly_trait_ref)],
             ) = (&type_param_path.segments[..], &bounds[..])
+                && let [ast::PathSegment { ident, args: None, id }] =
+                    &poly_trait_ref.trait_ref.path.segments[..]
                 && poly_trait_ref.modifiers == ast::TraitBoundModifiers::NONE
             {
-                if let [ast::PathSegment { ident, args: None, .. }] =
-                    &poly_trait_ref.trait_ref.path.segments[..]
-                {
-                    if ident.span == span {
-                        let Some(new_where_bound_predicate) =
-                            mk_where_bound_predicate(path, poly_trait_ref, ty)
-                        else {
-                            return false;
-                        };
-                        err.span_suggestion_verbose(
-                            *where_span,
-                            format!("constrain the associated type to `{ident}`"),
-                            where_bound_predicate_to_string(&new_where_bound_predicate),
-                            Applicability::MaybeIncorrect,
-                        );
+                if ident.span == span {
+                    let Some(partial_res) = self.r.partial_res_map.get(&id) else {
+                        return false;
+                    };
+                    if !matches!(partial_res.full_res(), Some(hir::def::Res::Def(..))) {
+                        return false;
                     }
-                    return true;
+
+                    let Some(new_where_bound_predicate) =
+                        mk_where_bound_predicate(path, poly_trait_ref, ty)
+                    else {
+                        return false;
+                    };
+                    err.span_suggestion_verbose(
+                        *where_span,
+                        format!("constrain the associated type to `{ident}`"),
+                        where_bound_predicate_to_string(&new_where_bound_predicate),
+                        Applicability::MaybeIncorrect,
+                    );
                 }
+                return true;
             }
         }
         false

tests/ui/traits/associated_type_bound/assoc-type-maybe-trait-147356.rs

@@ -0,0 +1,16 @@
+use std::str::FromStr;
+fn foo<T: FromStr>() -> T
+where
+    <T as FromStr>::Err: Debug, //~ ERROR expected trait
+{
+    "".parse().unwrap()
+}
+
+fn bar<T: FromStr>() -> T
+where
+    <T as FromStr>::Err: some_unknown_name, //~ ERROR cannot find trait `some_unknown_name` in this scope
+{
+    "".parse().unwrap()
+}
+
+fn main() {}

tests/ui/traits/associated_type_bound/assoc-type-maybe-trait-147356.stderr

@@ -0,0 +1,21 @@
+error[E0404]: expected trait, found derive macro `Debug`
+  --> $DIR/assoc-type-maybe-trait-147356.rs:4:26
+   |
+LL |     <T as FromStr>::Err: Debug,
+   |                          ^^^^^ not a trait
+   |
+help: consider importing this trait instead
+   |
+LL + use std::fmt::Debug;
+   |
+
+error[E0405]: cannot find trait `some_unknown_name` in this scope
+  --> $DIR/assoc-type-maybe-trait-147356.rs:11:26
+   |
+LL |     <T as FromStr>::Err: some_unknown_name,
+   |                          ^^^^^^^^^^^^^^^^^ not found in this scope
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0404, E0405.
+For more information about an error, try `rustc --explain E0404`.