add regression test

tests/ui/impl-trait/associated-impl-trait-type-into-emplacable.rs

@@ -0,0 +1,52 @@
+//! add regression test for <https://github.com/rust-lang/rust/issues/146514>.
+
+//@ check-pass
+
+#![feature(impl_trait_in_assoc_type)]
+
+use std::marker::PhantomData;
+
+struct Emp<T, F> {
+    phantom: PhantomData<(*const T, F)>,
+}
+
+impl<T, F> Emp<T, F> {
+    fn from_fn(_: F) -> Emp<T, F> {
+        loop {}
+    }
+
+    fn unsize(self) -> Emp<Slice, impl Sized> {
+        Emp::from_fn(|| ())
+    }
+}
+
+trait IntoEmplacable {
+    type Closure;
+
+    fn into_emplacable(self) -> Emp<Slice, Self::Closure>;
+}
+
+impl<F> IntoEmplacable for Emp<Arr, F> {
+    type Closure = impl Sized;
+
+    fn into_emplacable(self) -> Emp<Slice, Self::Closure> {
+        self.unsize()
+    }
+}
+
+impl<F> Into<Emp<Slice, <Emp<Arr, F> as IntoEmplacable>::Closure>> for Emp<Arr, F> {
+    fn into(self) -> Emp<Slice, <Emp<Arr, F> as IntoEmplacable>::Closure> {
+        self.into_emplacable()
+    }
+}
+
+fn box_new_with(_: Emp<Slice, impl Sized>) {}
+
+pub struct Arr;
+pub struct Slice;
+
+pub fn foo() {
+    let e: Emp<Arr, ()> = Emp { phantom: PhantomData };
+    box_new_with(e.into());
+}
+fn main() {}