Add regression test for #47814

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

Author: JohnTitor

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

@@ -0,0 +1,14 @@
+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!()
+    }
+}
+
+fn main() {}

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

@@ -0,0 +1,14 @@
+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
+