Replace parent substs of associated types with inference vars in borrow checker

compiler/rustc_mir/src/borrow_check/type_check/free_region_relations.rs

@@ -235,6 +235,7 @@ struct UniversalRegionRelationsBuilder<'this, 'tcx> {
 
 impl UniversalRegionRelationsBuilder<'cx, 'tcx> {
     crate fn create(mut self) -> CreateResult<'tcx> {
+        let tcx = self.infcx.tcx;
         let unnormalized_input_output_tys = self
             .universal_regions
             .unnormalized_input_tys
@@ -266,6 +267,9 @@ impl UniversalRegionRelationsBuilder<'cx, 'tcx> {
                             .delay_span_bug(DUMMY_SP, &format!("failed to normalize {:?}", ty));
                         (self.infcx.tcx.ty_error(), None)
                     });
+                // We need to replace bound regions in the substs of associated types (parent substs, not GATs)
+                // with inference vars, see issue #78450
+                let ty = self.universal_regions.indices.fold_to_region_vids(tcx, ty);
                 let constraints2 = self.add_implied_bounds(ty);
                 normalized_inputs_and_output.push(ty);
                 constraints1.into_iter().chain(constraints2)

compiler/rustc_mir/src/borrow_check/universal_regions.rs

@@ -30,7 +30,7 @@ use crate::borrow_check::nll::ToRegionVid;
 
 #[derive(Debug)]
 pub struct UniversalRegions<'tcx> {
-    indices: UniversalRegionIndices<'tcx>,
+    pub(crate) indices: UniversalRegionIndices<'tcx>,
 
     /// The vid assigned to `'static`
     pub fr_static: RegionVid,
@@ -162,7 +162,7 @@ impl<'tcx> DefiningTy<'tcx> {
 }
 
 #[derive(Debug)]
-struct UniversalRegionIndices<'tcx> {
+pub(crate) struct UniversalRegionIndices<'tcx> {
     /// For those regions that may appear in the parameter environment
     /// ('static and early-bound regions), we maintain a map from the
     /// `ty::Region` to the internal `RegionVid` we are using. This is

src/test/ui/type-alias-impl-trait/issue-78450.rs

@@ -0,0 +1,27 @@
+// check-pass
+
+#![feature(min_type_alias_impl_trait)]
+#![feature(type_alias_impl_trait)]
+//~^ WARNING: the feature `type_alias_impl_trait` is incomplete
+
+pub trait AssociatedImpl {
+    type ImplTrait;
+
+    fn f() -> Self::ImplTrait;
+}
+
+struct S<T>(T);
+
+trait Associated {
+    type A;
+}
+
+impl<'a, T: Associated<A = &'a ()>> AssociatedImpl for S<T> {
+    type ImplTrait = impl core::fmt::Debug;
+
+    fn f() -> Self::ImplTrait {
+        ()
+    }
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/issue-78450.stderr

@@ -0,0 +1,11 @@
+warning: the feature `type_alias_impl_trait` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/issue-78450.rs:4:12
+   |
+LL | #![feature(type_alias_impl_trait)]
+   |            ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
+
+warning: 1 warning emitted
+