Add test

Author: tiif

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

@@ -0,0 +1,33 @@
+//@ compile-flags: -Znext-solver
+
+// Test for https://github.com/rust-lang/trait-system-refactor-initiative/issues/235
+
+// These are cases where an opaque types become "stranded" due to
+// some errors. Make sure we don't ICE in either case.
+
+// Case 1: `impl Send` is stranded
+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`
+    ()
+}
+
+// Case 2: `Assoc = impl Trait` is stranded
+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
+}
+
+// Case 3: `impl Trait` is stranded
+fn ill_formed_string() -> String<impl Trait> {
+    //~^ ERROR struct takes 0 generic arguments but 1 generic argument was supplied
+   String::from("a string")
+}
+
+fn main() {}

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

@@ -0,0 +1,40 @@
+error: bound modifier `?` can only be applied to `Sized`
+  --> $DIR/stranded_opaque.rs:11:18
+   |
+LL | fn foo() -> impl ?Future<Output = impl Send> {
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0220]: associated type `Assoc` not found for `Trait`
+  --> $DIR/stranded_opaque.rs:21:28
+   |
+LL | fn produce() -> impl Trait<Assoc = impl Trait> {
+   |                            ^^^^^ associated type `Assoc` not found
+
+error[E0107]: struct takes 0 generic arguments but 1 generic argument was supplied
+  --> $DIR/stranded_opaque.rs:28:27
+   |
+LL | fn ill_formed_string() -> String<impl Trait> {
+   |                           ^^^^^^------------ help: remove the unnecessary generics
+   |                           |
+   |                           expected 0 generic arguments
+
+error: bound modifier `?` can only be applied to `Sized`
+  --> $DIR/stranded_opaque.rs:11:18
+   |
+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:21:28
+   |
+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 5 previous errors
+
+Some errors have detailed explanations: E0107, E0220.
+For more information about an error, try `rustc --explain E0107`.