Add regression test

src/test/ui/impl-trait/nested-return-type2.rs

@@ -0,0 +1,28 @@
+trait Duh {}
+
+impl Duh for i32 {}
+
+trait Trait {
+    type Assoc: Duh;
+}
+
+// the fact that `R` is the `::Output` projection on `F` causes
+// an intermediate inference var to be generated which is then later
+// compared against the actually found `Assoc` type.
+impl<R: Duh, F: FnMut() -> R> Trait for F {
+    type Assoc = R;
+}
+
+// The `impl Send` here is then later compared against the inference var
+// created, causing the inference var to be set to `impl Send` instead of
+// the hidden type. We already have obligations registered on the inference
+// var to make it uphold the `: Duh` bound on `Trait::Assoc`. The opaque
+// type does not implement `Duh`, even if its hidden type does. So we error out.
+fn foo() -> impl Trait<Assoc = impl Send> {
+    //~^ ERROR `impl Send: Duh` is not satisfied
+    //~| ERROR `impl Send: Duh` is not satisfied
+    || 42
+}
+
+fn main() {
+}

src/test/ui/impl-trait/nested-return-type2.stderr

@@ -0,0 +1,32 @@
+error[E0277]: the trait bound `impl Send: Duh` is not satisfied
+  --> $DIR/nested-return-type2.rs:21:13
+   |
+LL | fn foo() -> impl Trait<Assoc = impl Send> {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Duh` is not implemented for `impl Send`
+   |
+note: required because of the requirements on the impl of `Trait` for `[closure@$DIR/nested-return-type2.rs:24:5: 24:10]`
+  --> $DIR/nested-return-type2.rs:12:31
+   |
+LL | impl<R: Duh, F: FnMut() -> R> Trait for F {
+   |                               ^^^^^     ^
+
+error[E0277]: the trait bound `impl Send: Duh` is not satisfied
+  --> $DIR/nested-return-type2.rs:21:43
+   |
+LL |   fn foo() -> impl Trait<Assoc = impl Send> {
+   |  ___________________________________________^
+LL | |
+LL | |
+LL | |     || 42
+LL | | }
+   | |_^ the trait `Duh` is not implemented for `impl Send`
+   |
+note: required because of the requirements on the impl of `Trait` for `[closure@$DIR/nested-return-type2.rs:24:5: 24:10]`
+  --> $DIR/nested-return-type2.rs:12:31
+   |
+LL | impl<R: Duh, F: FnMut() -> R> Trait for F {
+   |                               ^^^^^     ^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.