Account for non-types in substs for opaque type error messages

Fixes #68368

Previously, I assumed that the substs contained only types, which caused
the computed index number to be wrong.

src/librustc_typeck/collect.rs

@@ -1673,8 +1673,15 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                     ty::Param(_) => true,
                     _ => false,
                 };
-                let bad_substs: Vec<_> =
-                    substs.types().enumerate().filter(|(_, ty)| !is_param(ty)).collect();
+                let bad_substs: Vec<_> = substs
+                    .iter()
+                    .enumerate()
+                    .filter_map(|(i, k)| {
+                        if let GenericArgKind::Type(ty) = k.unpack() { Some((i, ty)) } else { None }
+                    })
+                    .filter(|(_, ty)| !is_param(ty))
+                    .collect();
+
                 if !bad_substs.is_empty() {
                     let identity_substs = InternalSubsts::identity_for_item(self.tcx, self.def_id);
                     for (i, bad_subst) in bad_substs {

src/test/ui/type-alias-impl-trait/issue-68368-non-defining-use.rs

@@ -0,0 +1,13 @@
+// Regression test for issue #68368
+// Ensures that we don't ICE when emitting an error
+// for a non-defining use when lifetimes are involved
+
+#![feature(type_alias_impl_trait)]
+trait Trait<T> {}
+type Alias<'a, U> = impl Trait<U>; //~ ERROR could not find defining uses
+fn f<'a>() -> Alias<'a, ()> {}
+//~^ ERROR defining opaque type use does not fully define opaque type: generic parameter `U`
+
+fn main() {}
+
+impl Trait<()> for () {}

src/test/ui/type-alias-impl-trait/issue-68368-non-defining-use.stderr

@@ -0,0 +1,14 @@
+error: defining opaque type use does not fully define opaque type: generic parameter `U` is specified as concrete type `()`
+  --> $DIR/issue-68368-non-defining-use.rs:8:1
+   |
+LL | fn f<'a>() -> Alias<'a, ()> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: could not find defining uses
+  --> $DIR/issue-68368-non-defining-use.rs:7:1
+   |
+LL | type Alias<'a, U> = impl Trait<U>;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+