[WIP] Trait aliases: Imply default trait bounds

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -971,6 +971,33 @@ impl<'tcx> ItemCtxt<'tcx> {
     ) -> Vec<(ty::Clause<'tcx>, Span)> {
         let mut bounds = Vec::new();
 
+        if let PredicateFilter::All = filter {
+            for param in hir_generics.params {
+                match param.kind {
+                    hir::GenericParamKind::Lifetime { .. } => (),
+                    hir::GenericParamKind::Type { .. } => {
+                        let param_ty = self.lowerer().lower_ty_param(param.hir_id);
+                        self.lowerer().add_implicit_sizedness_bounds(
+                            &mut bounds,
+                            param_ty,
+                            &[],
+                            ImpliedBoundsContext::TyParam(param.def_id, hir_generics.predicates),
+                            param.span,
+                        );
+                        self.lowerer().add_default_traits(
+                            &mut bounds,
+                            param_ty,
+                            &[],
+                            ImpliedBoundsContext::TyParam(param.def_id, hir_generics.predicates),
+                            param.span,
+                        );
+                    }
+                    // FIXME(fmease): Relevant, too?
+                    hir::GenericParamKind::Const { .. } => {}
+                }
+            }
+        }
+
         for predicate in hir_generics.predicates {
             let hir_id = predicate.hir_id;
             let hir::WherePredicateKind::BoundPredicate(predicate) = predicate.kind else {

tests/ui/traits/alias/default-trait-bounds.rs

@@ -0,0 +1,25 @@
+// FIXME(fmease): Write a nice description!
+//                Write a nice description!
+//                Write a nice description!
+// issue: <https://github.com/rust-lang/rust/issues/152687>
+
+//@ check-pass
+
+#![feature(trait_alias)]
+#![feature(sized_hierarchy)] // only used for test case (B)
+
+trait A0<T: Sized> =;
+fn f0<T: ?Sized>() where (): A0<T> {}
+
+trait A1<T> =; // has default `T: Sized` bound
+fn f1<T: ?Sized>() where (): A1<T> {}
+
+//
+
+trait B0<T: std::marker::MetaSized> =;
+fn g0<T: std::marker::PointeeSized>() where (): B0<T> {}
+
+trait B1<T: ?Sized> =; // has a default `T: MetaSized` bound
+fn g1<T: std::marker::PointeeSized>() where (): B1<T> {}
+
+fn main() {}

tests/ui/traits/alias/more-default-trait-bounds.rs

@@ -0,0 +1,20 @@
+// FIXME(fmease): Write a nice description!
+//                Write a nice description!
+//                Write a nice description!
+// issue: <https://github.com/rust-lang/rust/issues/152687>
+
+//@ compile-flags: -Zexperimental-default-bounds
+//@ check-pass
+
+#![feature(trait_alias, more_maybe_bounds, lang_items, auto_traits)]
+
+#[lang = "default_trait1"]
+auto trait Mark {}
+
+trait A0<T: Mark> = ?Mark;
+fn f0<T: ?Mark>() where (): A0<T> {}
+
+trait A1<T> = ?Mark; // has a default `T: Mark` bound
+fn f1<T: ?Mark>() where (): A1<T> {}
+
+fn main() {}