add test using only trait bounds

Author: lcnr

tests/ui/traits/next-solver/cycles/coinduction/only-one-coinductive-step-needed-trait.current.stderr

@@ -0,0 +1,22 @@
+error[E0275]: overflow evaluating the requirement `u32: SendIndir<Foo<u32>>`
+  --> $DIR/only-one-coinductive-step-needed-trait.rs:24:5
+   |
+LL |     is_send::<Foo<u32>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |
+note: required for `Foo<u32>` to implement `Send`
+  --> $DIR/only-one-coinductive-step-needed-trait.rs:17:35
+   |
+LL | unsafe impl<T: SendIndir<Foo<T>>> Send for Foo<T> {}
+   |                -----------------  ^^^^     ^^^^^^
+   |                |
+   |                unsatisfied trait bound introduced here
+note: required by a bound in `is_send`
+  --> $DIR/only-one-coinductive-step-needed-trait.rs:22:15
+   |
+LL | fn is_send<T: Send>() {}
+   |               ^^^^ required by this bound in `is_send`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.

tests/ui/traits/next-solver/cycles/coinduction/only-one-coinductive-step-needed-trait.rs

@@ -0,0 +1,26 @@
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@[next] check-pass
+
+// #136824 changed cycles to be coinductive if they have at least
+// one productive step, causing this test to pass with the new solver.
+//
+// The cycle in the test is the following:
+// - `Foo<T>: Send`, impl requires
+// - `T: SendIndir<Foo<T>>`, impl requires
+// - `Foo<T>: Send` cycle
+//
+// The old solver treats this cycle as inductive due to the `T: SendIndir` step.
+
+struct Foo<T>(T);
+unsafe impl<T: SendIndir<Foo<T>>> Send for Foo<T> {}
+
+trait SendIndir<T> {}
+impl<T, U: Send> SendIndir<U> for T {}
+
+fn is_send<T: Send>() {}
+fn main() {
+    is_send::<Foo<u32>>();
+    //[current]~^ ERROR overflow evaluating the requirement `u32: SendIndir<Foo<u32>>`
+}