Add regression test for #80816

src/test/ui/inference/issue-80816.rs

@@ -0,0 +1,54 @@
+#![allow(unreachable_code)]
+
+use std::marker::PhantomData;
+use std::ops::Deref;
+use std::sync::Arc;
+
+pub struct Guard<T> {
+    _phantom: PhantomData<T>,
+}
+impl<T> Deref for Guard<T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        unimplemented!()
+    }
+}
+
+pub struct DirectDeref<T>(T);
+impl<T> Deref for DirectDeref<Arc<T>> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        unimplemented!()
+    }
+}
+
+pub trait Access<T> {
+    type Guard: Deref<Target = T>;
+    fn load(&self) -> Self::Guard {
+        unimplemented!()
+    }
+}
+impl<T, A: Access<T>, P: Deref<Target = A>> Access<T> for P {
+    //~^ NOTE: required for `Arc<ArcSwapAny<Arc<usize>>>` to implement `Access<_>`
+    type Guard = A::Guard;
+}
+impl<T> Access<T> for ArcSwapAny<T> {
+    //~^ NOTE: multiple `impl`s satisfying `ArcSwapAny<Arc<usize>>: Access<_>` found
+    type Guard = Guard<T>;
+}
+impl<T> Access<T> for ArcSwapAny<Arc<T>> {
+    type Guard = DirectDeref<Arc<T>>;
+}
+
+pub struct ArcSwapAny<T> {
+    _phantom_arc: PhantomData<T>,
+}
+
+pub fn foo() {
+    let s: Arc<ArcSwapAny<Arc<usize>>> = unimplemented!();
+    let guard: Guard<Arc<usize>> = s.load();
+    //~^ ERROR: type annotations needed
+    //~| HELP: try using a fully qualified path to specify the expected types
+}
+
+fn main() {}

src/test/ui/inference/issue-80816.stderr

@@ -0,0 +1,27 @@
+error[E0283]: type annotations needed
+  --> $DIR/issue-80816.rs:49:38
+   |
+LL |     let guard: Guard<Arc<usize>> = s.load();
+   |                                      ^^^^
+   |
+note: multiple `impl`s satisfying `ArcSwapAny<Arc<usize>>: Access<_>` found
+  --> $DIR/issue-80816.rs:35:1
+   |
+LL | impl<T> Access<T> for ArcSwapAny<T> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL | impl<T> Access<T> for ArcSwapAny<Arc<T>> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required for `Arc<ArcSwapAny<Arc<usize>>>` to implement `Access<_>`
+  --> $DIR/issue-80816.rs:31:45
+   |
+LL | impl<T, A: Access<T>, P: Deref<Target = A>> Access<T> for P {
+   |                                             ^^^^^^^^^     ^
+help: try using a fully qualified path to specify the expected types
+   |
+LL |     let guard: Guard<Arc<usize>> = <Arc<ArcSwapAny<Arc<usize>>> as Access<T>>::load(&s);
+   |                                    ++++++++++++++++++++++++++++++++++++++++++++++++++ ~
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0283`.