Make sure to insert Sized bound first into clauses list

compiler/rustc_hir_analysis/src/bounds.rs

@@ -54,14 +54,20 @@ impl<'tcx> Bounds<'tcx> {
         span: Span,
         polarity: ty::PredicatePolarity,
     ) {
-        self.clauses.push((
+        let clause = (
             trait_ref
                 .map_bound(|trait_ref| {
                     ty::ClauseKind::Trait(ty::TraitPredicate { trait_ref, polarity })
                 })
                 .to_predicate(tcx),
             span,
-        ));
+        );
+        // FIXME(-Znext-solver): We can likely remove this hack once the new trait solver lands.
+        if tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
+            self.clauses.insert(0, clause);
+        } else {
+            self.clauses.push(clause);
+        }
     }
 
     pub fn push_projection_bound(

tests/ui/methods/probe-overflow-due-to-sized-predicate-ordering.rs

@@ -0,0 +1,30 @@
+//@ check-pass
+// Regression test due to #123279
+
+pub trait Job: AsJob {
+    fn run_once(&self);
+}
+
+impl<F: Fn()> Job for F {
+    fn run_once(&self) {
+        todo!()
+    }
+}
+
+pub trait AsJob {}
+
+// Ensure that `T: Sized + Job` by reordering the explicit `Sized` to where
+// the implicit sized pred would go.
+impl<T: Job + Sized> AsJob for T {}
+
+pub struct LoopingJobService {
+    job: Box<dyn Job>,
+}
+
+impl Job for LoopingJobService {
+    fn run_once(&self) {
+        self.job.run_once()
+    }
+}
+
+fn main() {}