Use correct ImplSource for alias bounds

compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs

@@ -157,8 +157,7 @@ fn to_selection<'tcx>(
                 })
             }
             CandidateSource::BuiltinImpl(builtin) => ImplSource::Builtin(builtin, nested),
-            CandidateSource::ParamEnv(_) => ImplSource::Param(nested),
+            CandidateSource::ParamEnv(_) | CandidateSource::AliasBound => ImplSource::Param(nested),
-            CandidateSource::AliasBound => ImplSource::Builtin(BuiltinImplSource::Misc, nested),
             CandidateSource::CoherenceUnknowable => {
                 span_bug!(span, "didn't expect to select an unknowable candidate")
             }

tests/ui/traits/next-solver/select-alias-bound-as-param.rs

@@ -0,0 +1,13 @@
+//@ check-pass
+//@ compile-flags: -Znext-solver
+
+pub(crate) fn y() -> impl FnMut() {
+    || {}
+}
+
+pub(crate) fn x(a: (), b: ()) {
+    let x = ();
+    y()()
+}
+
+fn main() {}