Restore Opaque behavior to coherence check

src/test/ui/coherence/issue-99663-2.rs

@@ -0,0 +1,22 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+
+struct Outer<T: ?Sized> {
+    i: InnerSend<T>,
+}
+
+type InnerSend<T: ?Sized> = impl Send;
+
+fn constrain<T: ?Sized>() -> InnerSend<T> {
+    ()
+}
+
+trait SendMustNotImplDrop {}
+
+#[allow(drop_bounds)]
+impl<T: ?Sized + Send + Drop> SendMustNotImplDrop for T {}
+
+impl<T: ?Sized> SendMustNotImplDrop for Outer<T> {}
+
+fn main() {}

src/test/ui/coherence/issue-99663.rs

@@ -0,0 +1,22 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+
+struct Send<T> {
+    i: InnerSend<T>,
+}
+
+type InnerSend<T> = impl Sized;
+
+fn constrain<T>() -> InnerSend<T> {
+    ()
+}
+
+trait SendMustNotImplDrop {}
+
+#[allow(drop_bounds)]
+impl<T: Drop> SendMustNotImplDrop for T {}
+
+impl<T> SendMustNotImplDrop for Send<T> {}
+
+fn main() {}

src/test/ui/impl-trait/negative-reasoning.stderr

@@ -7,7 +7,7 @@ LL | impl<T: std::fmt::Debug> AnotherTrait for T {}
 LL | impl AnotherTrait for D<OpaqueType> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `D<OpaqueType>`
    |
-   = note: downstream crates may implement trait `std::fmt::Debug` for type `OpaqueType`
+   = note: upstream crates may add a new impl of trait `std::fmt::Debug` for type `OpaqueType` in future versions
 
 error: cannot implement trait on type alias impl trait
   --> $DIR/negative-reasoning.rs:19:25