fix: Resolve projection types before checking casts

crates/ide-diagnostics/src/handlers/invalid_cast.rs

@@ -1129,4 +1129,39 @@ fn main() {
         "#,
         );
     }
+
+    #[test]
+    fn regression_18682() {
+        check_diagnostics(
+            r#"
+//- minicore: coerce_unsized
+struct Flexible {
+    body: [u8],
+}
+
+trait Field {
+    type Type: ?Sized;
+}
+
+impl Field for Flexible {
+    type Type = [u8];
+}
+
+trait KnownLayout {
+    type MaybeUninit: ?Sized;
+}
+
+
+impl<T> KnownLayout for [T] {
+    type MaybeUninit = [T];
+}
+
+struct ZerocopyKnownLayoutMaybeUninit(<<Flexible as Field>::Type as KnownLayout>::MaybeUninit);
+
+fn test(ptr: *mut [u8]) -> *mut ZerocopyKnownLayoutMaybeUninit {
+    ptr as *mut _
+}
+"#,
+        );
+    }
 }