fix: avoid infinite recursion while lowering associated type bounds from supertraits

crates/hir-ty/src/lower_nextsolver.rs

@@ -1756,7 +1756,11 @@ fn named_associated_type_shorthand_candidates<'db, R>(
                 db,
                 GenericDefId::TraitId(trait_def_id),
                 PredicateFilter::SelfTrait,
-                |pred| pred == GenericDefId::TraitId(trait_def_id),
+                // We are likely in the midst of lowering generic predicates of `def`.
+                // So, if we allow `pred == def` we might fall into an infinite recursion.
+                // Actually, we have already checked for the case `pred == def` above as we started
+                // with a stack including `trait_id`
+                |pred| pred != def && pred == GenericDefId::TraitId(trait_def_id),
             )
             .0
             .deref()

crates/hir-ty/src/tests/regression.rs

@@ -2439,3 +2439,56 @@ pub fn null_mut<T: PointeeSized + Thin>() -> *mut T {
 "#,
     );
 }
+
+#[test]
+fn issue_20484() {
+    check_no_mismatches(
+        r#"
+struct Eth;
+
+trait FullBlockBody {
+    type Transaction;
+}
+
+impl FullBlockBody for () {
+    type Transaction = ();
+}
+
+trait NodePrimitives {
+    type BlockBody;
+    type SignedTx;
+}
+
+impl NodePrimitives for () {
+    type BlockBody = ();
+    type SignedTx = ();
+}
+
+impl NodePrimitives for Eth {
+    type BlockBody = ();
+    type SignedTx = ();
+}
+
+trait FullNodePrimitives
+where
+    Self: NodePrimitives<BlockBody: FullBlockBody<Transaction = Self::SignedTx>>,
+{
+}
+
+impl<T> FullNodePrimitives for T where
+    T: NodePrimitives<BlockBody: FullBlockBody<Transaction = Self::SignedTx>>,
+{
+}
+
+fn node<N>(_: N)
+where
+    N: FullNodePrimitives,
+{
+}
+
+fn main() {
+    node(Eth);
+}
+"#,
+    );
+}