diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index fe2691e9d4d..af90372b97c 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -3842,7 +3842,9 @@ fn note_function_argument_obligation<G: EmissionGuarantee>(
                             self.probe(|_| {
                                 match self
                                     .at(&ObligationCause::misc(expr.span, body_id), param_env)
-                                    .eq(DefineOpaqueTypes::No, expected, actual)
+                                    // Doesn't actually matter if we define opaque types here, this is just used for
+                                    // diagnostics, and the result is never kept around.
+                                    .eq(DefineOpaqueTypes::Yes, expected, actual)
                                 {
                                     Ok(_) => (), // We ignore nested obligations here for now.
                                     Err(err) => type_diffs.push(err),