Re-add issue-53092

ices/53092.rs

@@ -0,0 +1,31 @@
+#![feature(const_fn)]
+#![feature(const_fn_union)]
+#![feature(type_alias_impl_trait)]
+#![feature(untagged_unions)]
+
+const fn transmute<T, U>(from: T) -> U {
+    use std::mem::ManuallyDrop;
+
+    union Transform<FROM, INTO> {
+        from: ManuallyDrop<FROM>,
+        into: ManuallyDrop<INTO>,
+    }
+
+    let transformer = Transform {
+        from: ManuallyDrop::new(from),
+    };
+
+    unsafe { ManuallyDrop::into_inner(transformer.into) }
+}
+
+type Bug<T, U> = impl Fn(T) -> U + Copy;
+
+const CONST_BUG: Bug<u8, ()> = transmute(|_: u8| ());
+
+fn make_bug<T, U: From<T>>() -> Bug<T, U> {
+    |x| x.into()
+}
+
+fn main() {
+    let x = CONST_BUG(0);
+}