1 files changed, 45 insertions, 0 deletions
diff --git a/tests/ui/traits/new-solver/alias_eq_dont_use_normalizes_to_if_substs_eq.rs b/tests/ui/traits/new-solver/alias_eq_dont_use_normalizes_to_if_substs_eq.rs
new file mode 100644
index 000000000..fd5d0e3b1
--- /dev/null
+++ b/tests/ui/traits/new-solver/alias_eq_dont_use_normalizes_to_if_substs_eq.rs
@@ -0,0 +1,45 @@
+// compile-flags: -Ztrait-solver=next
+
+// check that when computing `alias-eq(<() as Foo<u16, T>>::Assoc, <() as Foo<?0, T>>::Assoc)`
+//  we do not infer `?0 = u8` via the `for<STOP> (): Foo<u8, STOP>` impl or `?0 = u16` by
+// relating substs as either could be a valid solution.
+
+trait Foo<T, STOP> {
+    type Assoc;
+}
+
+impl<STOP> Foo<u8, STOP> for ()
+where
+    (): Foo<u16, STOP>,
+{
+    type Assoc = <() as Foo<u16, STOP>>::Assoc;
+}
+
+impl Foo<u16, i8> for () {
+    type Assoc = u8;
+}
+
+impl Foo<u16, i16> for () {
+    type Assoc = u16;
+}
+
+fn output<T, U>() -> <() as Foo<T, U>>::Assoc
+where
+    (): Foo<T, U>,
+{
+    todo!()
+}
+
+fn incomplete<T>()
+where
+    (): Foo<u16, T>,
+{
+    // `<() as Foo<u16, STOP>>::Assoc == <() as Foo<_, STOP>>::Assoc`
+    let _: <() as Foo<u16, T>>::Assoc = output::<_, T>();
+    //~^ error: type annotations needed
+
+    // let _: <() as Foo<u16, T>>::Assoc = output::<u8, T>(); // OK
+    // let _: <() as Foo<u16, T>>::Assoc = output::<u16, T>(); // OK
+}
+
+fn main() {}