Add a test that used to take forever to compile

tests/ui/traits/new-solver/nested-obligations-with-bound-vars-gat.rs

@@ -0,0 +1,43 @@
+// check-pass
+// compile-flags: -Ztrait-solver=next
+// Issue 96230
+
+use std::fmt::Debug;
+
+trait Classic {
+    type Assoc;
+}
+
+trait Gat {
+    type Assoc<'a>;
+}
+
+struct Foo;
+
+impl Classic for Foo {
+    type Assoc = ();
+}
+
+impl Gat for Foo {
+    type Assoc<'i> = ();
+}
+
+fn classic_debug<T: Classic>(_: T)
+where
+    T::Assoc: Debug,
+{
+}
+
+fn gat_debug<T: Gat>(_: T)
+where
+    for<'a> T::Assoc<'a>: Debug,
+{
+}
+
+fn main() {
+    classic_debug::<Foo>(Foo); // fine
+    classic_debug(Foo); // fine
+
+    gat_debug::<Foo>(Foo); // fine
+    gat_debug(Foo); // boom
+}