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+A `#[marker]` trait contained an associated item.
+
+Erroneous code example:
+
+```compile_fail,E0714
+#![feature(marker_trait_attr)]
+#![feature(associated_type_defaults)]
+
+#[marker]
+trait MarkerConst {
+    const N: usize; // error!
+}
+
+fn main() {}
+```
+
+The items of marker traits cannot be overridden, so there's no need to have them
+when they cannot be changed per-type anyway.  If you wanted them for ergonomic
+reasons, consider making an extension trait instead.