Don't ICE when auto trait has assoc ty in old solver

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1202,6 +1202,12 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                             false
                         }
                     }
+                } else if tcx.trait_is_auto(trait_ref.def_id) {
+                    tcx.dcx().span_delayed_bug(
+                        tcx.def_span(obligation.predicate.def_id),
+                        "associated types not allowed on auto traits",
+                    );
+                    false
                 } else {
                     bug!("unexpected builtin trait with associated type: {trait_ref:?}")
                 }

tests/ui/auto-traits/assoc-ty.current.stderr

@@ -0,0 +1,40 @@
+error[E0380]: auto traits cannot have associated items
+  --> $DIR/assoc-ty.rs:10:10
+   |
+LL | auto trait Trait {
+   |            ----- auto traits cannot have associated items
+LL |
+LL |     type Output;
+   |     -----^^^^^^- help: remove these associated items
+
+error[E0658]: auto traits are experimental and possibly buggy
+  --> $DIR/assoc-ty.rs:8:1
+   |
+LL | / auto trait Trait {
+LL | |
+LL | |     type Output;
+LL | |
+LL | | }
+   | |_^
+   |
+   = note: see issue #13231 <https://github.com/rust-lang/rust/issues/13231> for more information
+   = help: add `#![feature(auto_traits)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0308]: mismatched types
+  --> $DIR/assoc-ty.rs:15:36
+   |
+LL |     let _: <() as Trait>::Output = ();
+   |            ---------------------   ^^ expected associated type, found `()`
+   |            |
+   |            expected due to this
+   |
+   = note: expected associated type `<() as Trait>::Output`
+                    found unit type `()`
+   = help: consider constraining the associated type `<() as Trait>::Output` to `()` or calling a method that returns `<() as Trait>::Output`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0308, E0380, E0658.
+For more information about an error, try `rustc --explain E0308`.

tests/ui/auto-traits/assoc-ty.next.stderr

@@ -0,0 +1,40 @@
+error[E0380]: auto traits cannot have associated items
+  --> $DIR/assoc-ty.rs:10:10
+   |
+LL | auto trait Trait {
+   |            ----- auto traits cannot have associated items
+LL |
+LL |     type Output;
+   |     -----^^^^^^- help: remove these associated items
+
+error[E0658]: auto traits are experimental and possibly buggy
+  --> $DIR/assoc-ty.rs:8:1
+   |
+LL | / auto trait Trait {
+LL | |
+LL | |     type Output;
+LL | |
+LL | | }
+   | |_^
+   |
+   = note: see issue #13231 <https://github.com/rust-lang/rust/issues/13231> for more information
+   = help: add `#![feature(auto_traits)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0308]: mismatched types
+  --> $DIR/assoc-ty.rs:15:36
+   |
+LL |     let _: <() as Trait>::Output = ();
+   |            ---------------------   ^^ types differ
+   |            |
+   |            expected due to this
+   |
+   = note: expected associated type `<() as Trait>::Output`
+                    found unit type `()`
+   = help: consider constraining the associated type `<() as Trait>::Output` to `()` or calling a method that returns `<() as Trait>::Output`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0308, E0380, E0658.
+For more information about an error, try `rustc --explain E0308`.

tests/ui/auto-traits/assoc-ty.rs

@@ -0,0 +1,17 @@
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
+
+// Tests that projection doesn't explode if we accidentally
+// put an associated type on an auto trait.
+
+auto trait Trait {
+    //~^ ERROR auto traits are experimental and possibly buggy
+    type Output;
+    //~^ ERROR auto traits cannot have associated items
+}
+
+fn main() {
+    let _: <() as Trait>::Output = ();
+    //~^ ERROR mismatched types
+}