Add additional tests for type alias impl trait coherence

src/test/ui/impl-trait/auto-trait.rs

@@ -0,0 +1,23 @@
+// Tests that type alias impls traits do not leak auto-traits for
+// the purposes of coherence checking
+#![feature(type_alias_impl_trait)]
+
+trait OpaqueTrait { }
+impl<T> OpaqueTrait for T { }
+type OpaqueType = impl OpaqueTrait;
+fn mk_opaque() -> OpaqueType { () }
+
+#[derive(Debug)]
+struct D<T>(T);
+
+trait AnotherTrait { }
+impl<T: Send> AnotherTrait for T { }
+
+// This is in error, because we cannot assume that `OpaqueType: !Send`.
+// (We treat opaque types as "foreign types" that could grow more impls
+// in the future.)
+impl AnotherTrait for D<OpaqueType> {
+    //~^ ERROR conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`
+}
+
+fn main() {}

src/test/ui/impl-trait/auto-trait.stderr

@@ -0,0 +1,12 @@
+error[E0119]: conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`:
+  --> $DIR/auto-trait.rs:19:1
+   |
+LL | impl<T: Send> AnotherTrait for T { }
+   | -------------------------------- first implementation here
+...
+LL | impl AnotherTrait for D<OpaqueType> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `D<OpaqueType>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.

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

@@ -0,0 +1,22 @@
+// Tests that we cannot assume that an opaque type does *not* implement some
+// other trait
+#![feature(type_alias_impl_trait)]
+
+trait OpaqueTrait { }
+impl<T> OpaqueTrait for T { }
+type OpaqueType = impl OpaqueTrait;
+fn mk_opaque() -> OpaqueType { () }
+
+#[derive(Debug)]
+struct D<T>(T);
+
+trait AnotherTrait { }
+impl<T: std::fmt::Debug> AnotherTrait for T { }
+
+
+// This is in error, because we cannot assume that `OpaqueType: !Debug`
+impl AnotherTrait for D<OpaqueType> {
+    //~^ ERROR conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`
+}
+
+fn main() {}

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

@@ -0,0 +1,14 @@
+error[E0119]: conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`:
+  --> $DIR/negative-reasoning.rs:18:1
+   |
+LL | impl<T: std::fmt::Debug> AnotherTrait for T { }
+   | ------------------------------------------- first implementation here
+...
+LL | impl AnotherTrait for D<OpaqueType> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `D<OpaqueType>`
+   |
+   = note: upstream crates may add a new impl of trait `std::fmt::Debug` for type `OpaqueType` in future versions
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.