handle late-bound vars from inner binders correctly and add test

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -2093,7 +2093,7 @@ impl<'a, 'tcx> ty::TypeFolder<'tcx> for RegionFolder<'a, 'tcx> {
     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
         let name = &mut self.name;
         let region = match *r {
-            ty::ReLateBound(db, br) => {
+            ty::ReLateBound(db, br) if db >= self.current_index => {
                 *self.region_map.entry(br).or_insert_with(|| name(Some(db), self.current_index, br))
             }
             ty::RePlaceholder(ty::PlaceholderRegion { name: kind, .. }) => {

src/test/ui/lifetimes/nested-binder-print.rs

@@ -0,0 +1,10 @@
+struct TwoLt<'a, 'b>(&'a (), &'b ());
+type Foo<'a> = fn(TwoLt<'_, 'a>);
+
+fn foo() {
+    let y: for<'a> fn(Foo<'a>);
+    let x: u32 = y;
+    //~^ ERROR mismatched types
+}
+
+fn main() {}

src/test/ui/lifetimes/nested-binder-print.stderr

@@ -0,0 +1,14 @@
+error[E0308]: mismatched types
+  --> $DIR/nested-binder-print.rs:6:18
+   |
+LL |     let x: u32 = y;
+   |            ---   ^ expected `u32`, found fn pointer
+   |            |
+   |            expected due to this
+   |
+   = note:    expected type `u32`
+           found fn pointer `for<'a> fn(for<'b> fn(TwoLt<'b, 'a>))`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.