Auto merge of #146885 - lcnr:method-selection-opaques, r=BoxyUwU

support opaque types in method selection

See my notes in https://hackmd.io/4ILASx3mQ3u_gW9r1JyqCw.

This PR builds on https://github.com/rust-lang/rust/pull/145993 and allows not-yet defined opaque types as self types in the `method_autoderef_chain`.

E.g. for `Box<impl Deref<impl Foo>>` this results in the autoderef chain `Box<impl Deref> -> ?deref_hidden_ty -> ?foo_hidden_ty`. Method selection stays ambiguous if the final autoderef step is still an infer var unless that var is an opaque.

TODO: treating opaques as rigid jank.

r? `@BoxyUwU`

1 files changed, 27 insertions, 0 deletions

diff --git a/tests/ui/impl-trait/method/method-resolution3.rs b/tests/ui/impl-trait/method/method-resolution3.rs
new file mode 100644
index 00000000000..8c47ef4fc75
--- /dev/null
+++ b/tests/ui/impl-trait/method/method-resolution3.rs
@@ -0,0 +1,27 @@
+//! Check that we consider `Bar<impl Sized>` to successfully unify
+//! with both `Bar<u32>` and `Bar<i32>` (in isolation), so we bail
+//! out with ambiguity.
+
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+
+struct Bar<T>(T);
+
+impl Bar<u32> {
+    fn bar(self) {}
+}
+
+impl Bar<i32> {
+    fn bar(self) {}
+}
+
+fn foo(x: bool) -> Bar<impl Sized> {
+    if x {
+        let x = foo(false);
+        x.bar();
+        //~^ ERROR: multiple applicable items in scope
+    }
+    todo!()
+}
+
+fn main() {}