From 23cb89ea3810bf63a70b9e7b473c046457982179 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Sat, 16 Jul 2022 18:35:56 -0700
Subject: [PATCH] Do not constraint TAITs when checking impl/trait item
 compatibility

---
 .../rustc_typeck/src/check/compare_method.rs  | 15 +++++++++++
 .../issues/issue-99348-impl-compatibility.rs  | 26 +++++++++++++++++++
 .../issue-99348-impl-compatibility.stderr     | 15 +++++++++++
 3 files changed, 56 insertions(+)
 create mode 100644 src/test/ui/impl-trait/issues/issue-99348-impl-compatibility.rs
 create mode 100644 src/test/ui/impl-trait/issues/issue-99348-impl-compatibility.stderr

diff --git a/compiler/rustc_typeck/src/check/compare_method.rs b/compiler/rustc_typeck/src/check/compare_method.rs
index a53217ef818..6ae17fc6176 100644
--- a/compiler/rustc_typeck/src/check/compare_method.rs
+++ b/compiler/rustc_typeck/src/check/compare_method.rs
@@ -1505,6 +1505,21 @@ pub fn check_type_bounds<'tcx>(
             &outlives_environment,
         );
 
+        let constraints = infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
+        for (key, value) in constraints {
+            infcx
+                .report_mismatched_types(
+                    &ObligationCause::misc(
+                        value.hidden_type.span,
+                        tcx.hir().local_def_id_to_hir_id(impl_ty.def_id.expect_local()),
+                    ),
+                    tcx.mk_opaque(key.def_id, key.substs),
+                    value.hidden_type.ty,
+                    TypeError::Mismatch,
+                )
+                .emit();
+        }
+
         Ok(())
     })
 }
diff --git a/src/test/ui/impl-trait/issues/issue-99348-impl-compatibility.rs b/src/test/ui/impl-trait/issues/issue-99348-impl-compatibility.rs
new file mode 100644
index 00000000000..d29a82f76a7
--- /dev/null
+++ b/src/test/ui/impl-trait/issues/issue-99348-impl-compatibility.rs
@@ -0,0 +1,26 @@
+#![feature(type_alias_impl_trait)]
+
+struct Concrete;
+
+type Tait = impl Sized;
+
+impl Foo for Concrete {
+    type Item = Concrete;
+    //~^ mismatched types
+}
+
+impl Bar for Concrete {
+    type Other = Tait;
+}
+
+trait Foo {
+    type Item: Bar<Other = Self>;
+}
+
+trait Bar {
+    type Other;
+}
+
+fn tait() -> Tait {}
+
+fn main() {}
diff --git a/src/test/ui/impl-trait/issues/issue-99348-impl-compatibility.stderr b/src/test/ui/impl-trait/issues/issue-99348-impl-compatibility.stderr
new file mode 100644
index 00000000000..a25f0cd8761
--- /dev/null
+++ b/src/test/ui/impl-trait/issues/issue-99348-impl-compatibility.stderr
@@ -0,0 +1,15 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-99348-impl-compatibility.rs:8:17
+   |
+LL | type Tait = impl Sized;
+   |             ---------- the expected opaque type
+...
+LL |     type Item = Concrete;
+   |                 ^^^^^^^^ types differ
+   |
+   = note: expected opaque type `Tait`
+                   found struct `Concrete`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.