Don't project to RPITIT that has no default value

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -344,8 +344,8 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::EarlyBinder<Ty<'_>>
                     in_trait,
                     ..
                 }) => {
-                    if in_trait {
+                    if in_trait && !tcx.impl_defaultness(owner).has_value() {
-                        assert!(tcx.impl_defaultness(owner).has_value());
+                        span_bug!(tcx.def_span(def_id), "tried to get type of this RPITIT with no definition");
                     }
                     find_opaque_ty_constraints_for_rpit(tcx, def_id, owner)
                 }

compiler/rustc_trait_selection/src/traits/project.rs

@@ -2200,7 +2200,8 @@ fn confirm_impl_trait_in_trait_candidate<'tcx>(
         Err(guar) => return Progress::error(tcx, guar),
     };
     // We don't support specialization for RPITITs anyways... yet.
-    if !leaf_def.is_final() {
+    // Also don't try to project to an RPITIT that has no value
+    if !leaf_def.is_final() || !leaf_def.item.defaultness(tcx).has_value() {
         return Progress { term: tcx.ty_error_misc().into(), obligations };
     }
 

tests/ui/impl-trait/in-trait/dont-project-to-rpitit-with-no-value.rs

@@ -0,0 +1,16 @@
+#![feature(return_position_impl_trait_in_trait)]
+//~^ WARN the feature `return_position_impl_trait_in_trait` is incomplete
+
+trait MyTrait {
+    fn foo(&self) -> impl Sized;
+    fn bar(&self) -> impl Sized;
+}
+
+impl MyTrait for i32 {
+//~^ ERROR not all trait items implemented, missing: `foo`
+    fn bar(&self) -> impl Sized {
+        self.foo()
+    }
+}
+
+fn main() {}

tests/ui/impl-trait/in-trait/dont-project-to-rpitit-with-no-value.stderr

@@ -0,0 +1,21 @@
+warning: the feature `return_position_impl_trait_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/dont-project-to-rpitit-with-no-value.rs:1:12
+   |
+LL | #![feature(return_position_impl_trait_in_trait)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0046]: not all trait items implemented, missing: `foo`
+  --> $DIR/dont-project-to-rpitit-with-no-value.rs:9:1
+   |
+LL |     fn foo(&self) -> impl Sized;
+   |     ---------------------------- `foo` from trait
+...
+LL | impl MyTrait for i32 {
+   | ^^^^^^^^^^^^^^^^^^^^ missing `foo` in implementation
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0046`.