add test for 47814

not sure if the issue should actually get closed though, hm

r? @compiler-errors

src/test/ui/associated-types/issue-47814.rs

+struct ArpIPv4<'a> {
+    s: &'a u8
+}
+
+impl<'a> ArpIPv4<'a> {
+    const LENGTH: usize = 20;
+
+    pub fn to_buffer() -> [u8; Self::LENGTH] { //~ ERROR generic `Self` types are currently not permitted in anonymous constants
+        unimplemented!()
+    }
+}
+
+pub fn main() {}

src/test/ui/associated-types/issue-47814.stderr

+error: generic `Self` types are currently not permitted in anonymous constants
+  --> $DIR/issue-47814.rs:8:32
+   |
+LL |     pub fn to_buffer() -> [u8; Self::LENGTH] {
+   |                                ^^^^
+   |
+note: not a concrete type
+  --> $DIR/issue-47814.rs:5:10
+   |
+LL | impl<'a> ArpIPv4<'a> {
+   |          ^^^^^^^^^^^
+
+error: aborting due to previous error
+