From cfd47afa1964343afbd963bd713c4bfa1022f80b Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Fri, 26 Aug 2022 04:39:41 +0000
Subject: [PATCH] Erase late bound regions before comparing types in
 suggest_dereferences

---
 .../src/traits/error_reporting/suggestions.rs | 12 ++++--
 .../generic-associated-types/issue-101020.rs  | 37 +++++++++++++++++++
 .../issue-101020.stderr                       | 25 +++++++++++++
 3 files changed, 70 insertions(+), 4 deletions(-)
 create mode 100644 src/test/ui/generic-associated-types/issue-101020.rs
 create mode 100644 src/test/ui/generic-associated-types/issue-101020.stderr

diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index a93f9ec0397d..54f01577c5e5 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -690,13 +690,17 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 real_trait_pred = parent_trait_pred;
             }
 
-            // Skipping binder here, remapping below
-            let real_ty = real_trait_pred.self_ty().skip_binder();
-            if self.can_eq(obligation.param_env, real_ty, arg_ty).is_err() {
+            let real_ty = real_trait_pred.self_ty();
+            // We `erase_late_bound_regions` here because `make_subregion` does not handle
+            // `ReLateBound`, and we don't particularly care about the regions.
+            if self
+                .can_eq(obligation.param_env, self.tcx.erase_late_bound_regions(real_ty), arg_ty)
+                .is_err()
+            {
                 continue;
             }
 
-            if let ty::Ref(region, base_ty, mutbl) = *real_ty.kind() {
+            if let ty::Ref(region, base_ty, mutbl) = *real_ty.skip_binder().kind() {
                 let mut autoderef = Autoderef::new(
                     self,
                     obligation.param_env,
diff --git a/src/test/ui/generic-associated-types/issue-101020.rs b/src/test/ui/generic-associated-types/issue-101020.rs
new file mode 100644
index 000000000000..51cabe21e629
--- /dev/null
+++ b/src/test/ui/generic-associated-types/issue-101020.rs
@@ -0,0 +1,37 @@
+#![feature(generic_associated_types)]
+
+pub trait LendingIterator {
+    type Item<'a>
+    where
+        Self: 'a;
+
+    fn consume<F>(self, _f: F)
+    where
+        Self: Sized,
+        for<'a> Self::Item<'a>: FuncInput<'a, Self::Item<'a>>,
+    {
+    }
+}
+
+impl<I: LendingIterator + ?Sized> LendingIterator for &mut I {
+    type Item<'a> = I::Item<'a> where Self: 'a;
+}
+struct EmptyIter;
+impl LendingIterator for EmptyIter {
+    type Item<'a> = &'a mut () where Self:'a;
+}
+pub trait FuncInput<'a, F>
+where
+    F: Foo<Self>,
+    Self: Sized,
+{
+}
+impl<'a, T, F: 'a> FuncInput<'a, F> for T where F: Foo<T> {}
+trait Foo<T> {}
+
+fn map_test() {
+    (&mut EmptyIter).consume(());
+    //~^ ERROR the trait bound `for<'a> &'a mut (): Foo<&'a mut ()>` is not satisfied
+}
+
+fn main() {}
diff --git a/src/test/ui/generic-associated-types/issue-101020.stderr b/src/test/ui/generic-associated-types/issue-101020.stderr
new file mode 100644
index 000000000000..7fde89eb75e5
--- /dev/null
+++ b/src/test/ui/generic-associated-types/issue-101020.stderr
@@ -0,0 +1,25 @@
+error[E0277]: the trait bound `for<'a> &'a mut (): Foo<&'a mut ()>` is not satisfied
+  --> $DIR/issue-101020.rs:33:5
+   |
+LL |     (&mut EmptyIter).consume(());
+   |     ^^^^^^^^^^^^^^^^ ------- required by a bound introduced by this call
+   |     |
+   |     the trait `for<'a> Foo<&'a mut ()>` is not implemented for `&'a mut ()`
+   |
+note: required for `&'a mut ()` to implement `for<'a> FuncInput<'a, &'a mut ()>`
+  --> $DIR/issue-101020.rs:29:20
+   |
+LL | impl<'a, T, F: 'a> FuncInput<'a, F> for T where F: Foo<T> {}
+   |                    ^^^^^^^^^^^^^^^^     ^
+note: required by a bound in `LendingIterator::consume`
+  --> $DIR/issue-101020.rs:11:33
+   |
+LL |     fn consume<F>(self, _f: F)
+   |        ------- required by a bound in this
+...
+LL |         for<'a> Self::Item<'a>: FuncInput<'a, Self::Item<'a>>,
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `LendingIterator::consume`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.