Add a regression test for #72207

tests/ui/impl-trait/associated-type-fn-bound-issue-72207.rs

@@ -0,0 +1,57 @@
+//@ check-pass
+//@ compile-flags: --crate-type=lib
+
+#![allow(dead_code)]
+
+use std::marker::PhantomData;
+
+pub struct XImpl<T, E, F2, F1>
+where
+    F2: Fn(E),
+{
+    f1: F1,
+    f2: F2,
+    _ghost: PhantomData<(T, E)>,
+}
+
+pub trait X<T>: Sized {
+    type F1;
+    type F2: Fn(Self::E);
+    type E;
+
+    fn and<NewF1, NewF1Generator>(self, f: NewF1Generator) -> XImpl<T, Self::E, Self::F2, NewF1>
+    where
+        NewF1Generator: FnOnce(Self::F1) -> NewF1;
+}
+
+impl<T, E, F2, F1> X<T> for XImpl<T, E, F2, F1>
+where
+    F2: Fn(E),
+{
+    type E = E;
+    type F2 = F2;
+    type F1 = F1;
+
+    fn and<NewF1, NewF1Generator>(self, f: NewF1Generator) -> XImpl<T, E, F2, NewF1>
+    where
+        NewF1Generator: FnOnce(F1) -> NewF1,
+    {
+        XImpl {
+            f1: f(self.f1),
+            f2: self.f2,
+            _ghost: PhantomData,
+        }
+    }
+}
+
+fn f() -> impl X<(), E = ()> {
+    XImpl {
+        f1: || (),
+        f2: |()| (),
+        _ghost: PhantomData,
+    }
+}
+
+fn f2() -> impl X<(), E = ()> {
+    f().and(|rb| rb)
+}