Normalize struct types in confirm_builtin_unsize_candidate

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -1039,9 +1039,25 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     return Err(Unimplemented);
                 }
 
-                // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`.
-                let source_tail = tail_field_ty.subst(tcx, substs_a);
-                let target_tail = tail_field_ty.subst(tcx, substs_b);
+                // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
+                // normalizing in the process, since `type_of` returns something directly from
+                // astconv (which means it's un-normalized).
+                let source_tail = normalize_with_depth_to(
+                    self,
+                    obligation.param_env,
+                    obligation.cause.clone(),
+                    obligation.recursion_depth + 1,
+                    tail_field_ty.subst(tcx, substs_a),
+                    &mut nested,
+                );
+                let target_tail = normalize_with_depth_to(
+                    self,
+                    obligation.param_env,
+                    obligation.cause.clone(),
+                    obligation.recursion_depth + 1,
+                    tail_field_ty.subst(tcx, substs_b),
+                    &mut nested,
+                );
 
                 // Check that the source struct with the target's
                 // unsizing parameters is equal to the target.

src/test/ui/unsized/issue-75899-but-gats.rs

@@ -0,0 +1,21 @@
+// check-pass
+
+use std::fmt::Debug;
+use std::marker::PhantomData;
+
+trait Foo {
+    type Gat<'a>: ?Sized where Self: 'a;
+}
+
+struct Bar<'a, T: Foo + 'a>(T::Gat<'a>);
+
+struct Baz<T: ?Sized>(PhantomData<T>);
+
+impl<T: ?Sized> Foo for Baz<T> {
+    type Gat<'a> = T where Self: 'a;
+}
+
+fn main() {
+    let x = Bar::<'_, Baz<()>>(());
+    let y: &Bar<'_, Baz<dyn Debug>> = &x;
+}

src/test/ui/unsized/issue-75899.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+trait Trait {}
+impl<T> Trait for T {}
+
+trait Noop {
+    type Assoc: ?Sized;
+}
+impl<T: ?Sized> Noop for T {
+    type Assoc = T;
+}
+
+struct NoopNewtype<T: ?Sized + Noop>(T::Assoc);
+fn coerce_newtype<T: Trait>(x: &NoopNewtype<T>) -> &NoopNewtype<dyn Trait + '_> {
+    x
+}
+
+fn main() {}