rework min_choice algorithm of member constraints

See the inline comments for the description of the new algorithm.

1 files changed, 29 insertions, 0 deletions

diff --git a/tests/ui/nll/member-constraints/nested-impl-trait-pass.rs b/tests/ui/nll/member-constraints/nested-impl-trait-pass.rs
new file mode 100644
index 00000000000..15540cb460e
--- /dev/null
+++ b/tests/ui/nll/member-constraints/nested-impl-trait-pass.rs
@@ -0,0 +1,29 @@
+// Nested impl-traits can impose different member constraints on the same region variable.
+
+// check-pass
+
+trait Cap<'a> {}
+impl<T> Cap<'_> for T {}
+
+// Assuming the hidden type is `[&'_#15r u8; 1]`, we have two distinct member constraints:
+// - '_#15r member ['static, 'a, 'b] // from outer impl-trait
+// - '_#15r member ['static, 'a]     // from inner impl-trait
+// To satisfy both we can only choose 'a.
+fn pass_early_bound<'s, 'a, 'b>(a: &'s u8) -> impl IntoIterator<Item = impl Cap<'a>> + Cap<'b>
+where
+    's: 'a,
+    's: 'b,
+{
+    [a]
+}
+
+// Same as the above but with late-bound regions.
+fn pass_late_bound<'s, 'a, 'b>(
+    a: &'s u8,
+    _: &'a &'s u8,
+    _: &'b &'s u8,
+) -> impl IntoIterator<Item = impl Cap<'a>> + Cap<'b> {
+    [a]
+}
+
+fn main() {}