regression test for alias-relate changes in lub

tests/ui/traits/next-solver/generalize/relate-alias-in-lub.rs

@@ -0,0 +1,30 @@
+//@ revisions: old next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@ check-pass
+// Reproduces https://github.com/rust-lang/rust/pull/151746#issuecomment-3822930803.
+//
+// The change we tried to make there caused relating a type variable with an alias inside lub,
+// In 5bd20bbd0ba6c0285664e55a1ffc677d7487c98b, we moved around code
+// that adds an alias-relate predicate to be earlier, from one shared codepath into several
+// distinct code paths. However, we forgot one codepath, through lub, causing an ICE in serde.
+// In the end we dropped said commit, but the reproducer is still a useful as test.
+
+use std::marker::PhantomData;
+
+pub trait Trait {
+    type Error;
+}
+
+pub struct Struct<E>(PhantomData<E>);
+
+impl Trait for () {
+    type Error = ();
+}
+
+fn main() {
+    let _: Struct<<() as Trait>::Error> = match loop {} {
+        b => loop {},
+        a => Struct(PhantomData),
+    };
+}