add test where we ignore hr implied bounds

tests/ui/regions/higher-ranked-implied.rs

@@ -0,0 +1,14 @@
+// FIXME: This test should pass as the first two fields add implied bounds that
+// `'a` is equal to `'b` while the last one should simply use that fact. With
+// the current implementation this errors. We have to be careful as implied bounds
+// are only sound if they're also correctly checked.
+
+struct Inv<T>(*mut T); // `T` is invariant.
+type A = for<'a, 'b> fn(Inv<&'a &'b ()>, Inv<&'b &'a ()>, Inv<&'a ()>);
+type B = for<'a, 'b> fn(Inv<&'a &'b ()>, Inv<&'b &'a ()>, Inv<&'b ()>);
+
+fn main() {
+    let x: A = |_, _, _| ();
+    let y: B = x; //~ ERROR mismatched types
+    let _: A = y; //~ ERROR mismatched types
+}

tests/ui/regions/higher-ranked-implied.stderr

@@ -0,0 +1,21 @@
+error[E0308]: mismatched types
+  --> $DIR/higher-ranked-implied.rs:12:16
+   |
+LL |     let y: B = x;
+   |                ^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'a, 'b> fn(Inv<&'a &'b ()>, Inv<&'b &'a ()>, Inv<&'b ()>)`
+              found fn pointer `for<'a, 'b> fn(Inv<&'a &'b ()>, Inv<&'b &'a ()>, Inv<&'a ()>)`
+
+error[E0308]: mismatched types
+  --> $DIR/higher-ranked-implied.rs:13:16
+   |
+LL |     let _: A = y;
+   |                ^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'a, 'b> fn(Inv<&'a &'b ()>, Inv<&'b &'a ()>, Inv<&'a ()>)`
+              found fn pointer `for<'a, 'b> fn(Inv<&'a &'b ()>, Inv<&'b &'a ()>, Inv<&'b ()>)`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.