add tests

Author: lcnr

tests/ui/nll/member-constraints/nested-impl-trait-pass.rs

@@ -8,7 +8,9 @@ impl<T> Cap<'_> for T {}
 // Assuming the hidden type is `[&'?15 u8; 1]`, we have two distinct member constraints:
 // - '?15 member ['static, 'a, 'b] // from outer impl-trait
 // - '?15 member ['static, 'a]     // from inner impl-trait
-// To satisfy both we can only choose 'a.
+// To satisfy both we can only choose 'a. Concretely, the applying the first member constraint
+// requires ?15 to outlive at least 'b while the second requires ?15 to outlive 'a. As
+// 'a outlives 'b we end up with 'a as the final member region.
 fn pass_early_bound<'s, 'a, 'b>(a: &'s u8) -> impl IntoIterator<Item = impl Cap<'a>> + Cap<'b>
 where
     's: 'a,

tests/ui/nll/member-constraints/reject-choice-due-to-prev-constraint.rs

@@ -0,0 +1,31 @@
+//@ check-pass
+
+// We've got `'0 member ['a, 'b, 'static]` and `'1 member ['a, 'b, 'static]`.
+//
+// As '0 gets outlived by 'a - its "upper bound" -  the only applicable choice
+// region is 'a.
+//
+// '1 has to outlive 'b so the only applicabel choice regions are 'b and 'static.
+// Considering this member constraint by itself would choose 'b as it is the
+// smaller of the two regions.
+//
+// However, this is only the case when ignoring the member constraint on '0.
+// After applying this constraint and requiring '0 to outlive 'a. As '1 outlives
+// '0, the region 'b is no longer an applicable choice region for '1 as 'b does
+// does not outlive 'a. We would therefore choose 'static.
+//
+// This means applying member constraints is order dependent. We handle this by
+// first applying member constraints for regions 'x and then consider the resulting
+// constraints when applying member constraints for regions 'y with 'y: 'x.
+fn with_constraints<'r0, 'r1, 'a, 'b>() -> *mut (&'r0 (), &'r1 ())
+where
+    'r1: 'r0,
+    'a: 'r0,
+    'r1: 'b,
+{
+    loop {}
+}
+fn foo<'a, 'b>() -> impl Sized + use<'a, 'b> {
+    with_constraints::<'_, '_, 'a, 'b>()
+}
+fn main() {}