Make sure to instantiate placeholders correctly in old solver

compiler/rustc_infer/src/infer/canonical/query_response.rs

@@ -483,7 +483,13 @@ impl<'tcx> InferCtxt<'tcx> {
         let result_subst = CanonicalVarValues {
             var_values: self.tcx.mk_args_from_iter(
                 query_response.variables.iter().enumerate().map(|(index, info)| {
-                    if info.is_existential() {
+                    if info.universe() != ty::UniverseIndex::ROOT {
+                        // A variable from inside a binder of the query. While ideally these shouldn't
+                        // exist at all, we have to deal with them for now.
+                        self.instantiate_canonical_var(cause.span, info, |u| {
+                            universe_map[u.as_usize()]
+                        })
+                    } else if info.is_existential() {
                         match opt_values[BoundVar::new(index)] {
                             Some(k) => k,
                             None => self.instantiate_canonical_var(cause.span, info, |u| {
@@ -491,9 +497,11 @@ impl<'tcx> InferCtxt<'tcx> {
                             }),
                         }
                     } else {
-                        self.instantiate_canonical_var(cause.span, info, |u| {
-                            universe_map[u.as_usize()]
-                        })
+                        // For placeholders which were already part of the input, we simply map this
+                        // universal bound variable back the placeholder of the input.
+                        opt_values[BoundVar::new(index)].expect(
+                            "expected placeholder to be unified with itself during response",
+                        )
                     }
                 }),
             ),

tests/ui/traits/non_lifetime_binders/unifying-placeholders-in-query-response.current.stderr

@@ -0,0 +1,11 @@
+warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/unifying-placeholders-in-query-response.rs:5:12
+   |
+LL | #![feature(non_lifetime_binders)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/traits/non_lifetime_binders/unifying-placeholders-in-query-response.next.stderr

@@ -0,0 +1,11 @@
+warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/unifying-placeholders-in-query-response.rs:5:12
+   |
+LL | #![feature(non_lifetime_binders)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/traits/non_lifetime_binders/unifying-placeholders-in-query-response.rs

@@ -0,0 +1,27 @@
+// revisions: current next
+//[next] compile-flags: -Znext-solver
+// check-pass
+
+#![feature(non_lifetime_binders)]
+//~^ WARN the feature `non_lifetime_binders` is incomplete
+
+pub trait Foo<T: ?Sized> {
+    type Bar<K: ?Sized>: ?Sized;
+}
+
+impl Foo<usize> for () {
+    type Bar<K: ?Sized> = K;
+}
+
+pub fn f<T1, T2>(a: T1, b: T2)
+where
+    T1: for<T> Foo<usize, Bar<T> = T>,
+    T2: for<T> Foo<usize, Bar<T> = <T1 as Foo<usize>>::Bar<T>>,
+{
+}
+
+fn it_works() {
+    f((), ());
+}
+
+fn main() {}