Add test

Author: tiif

tests/ui/traits/next-solver/opaques/stranded_opaque.rs

@@ -0,0 +1,25 @@
+//@ compile-flags: -Znext-solver
+
+// Test for https://github.com/rust-lang/trait-system-refactor-initiative/issues/235
+
+// These are cases where the inner opaque types become "stranded" due to
+// errors of outer opaque types. Make sure we don't ICE in both cases.
+
+use std::future::Future;
+
+fn foo() -> impl ?Future<Output = impl Send> {
+    //~^ ERROR: bound modifier `?` can only be applied to `Sized`
+    //~| ERROR: bound modifier `?` can only be applied to `Sized`
+    ()
+}
+
+trait Trait {}
+impl Trait for i32 {}
+
+fn produce() -> impl Trait<Assoc = impl Trait> {
+    //~^ ERROR associated type `Assoc` not found for `Trait`
+    //~| ERROR associated type `Assoc` not found for `Trait`
+    16
+}
+
+fn main() {}

tests/ui/traits/next-solver/opaques/stranded_opaque.stderr

@@ -0,0 +1,31 @@
+error: bound modifier `?` can only be applied to `Sized`
+  --> $DIR/stranded_opaque.rs:10:18
+   |
+LL | fn foo() -> impl ?Future<Output = impl Send> {
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0220]: associated type `Assoc` not found for `Trait`
+  --> $DIR/stranded_opaque.rs:19:28
+   |
+LL | fn produce() -> impl Trait<Assoc = impl Trait> {
+   |                            ^^^^^ associated type `Assoc` not found
+
+error: bound modifier `?` can only be applied to `Sized`
+  --> $DIR/stranded_opaque.rs:10:18
+   |
+LL | fn foo() -> impl ?Future<Output = impl Send> {
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0220]: associated type `Assoc` not found for `Trait`
+  --> $DIR/stranded_opaque.rs:19:28
+   |
+LL | fn produce() -> impl Trait<Assoc = impl Trait> {
+   |                            ^^^^^ associated type `Assoc` not found
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0220`.