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+An `impl` for a `#[marker]` trait tried to override an associated item.
+
+Erroneous code example:
+
+```compile_fail,E0715
+#![feature(marker_trait_attr)]
+
+#[marker]
+trait Marker {
+    const N: usize = 0;
+    fn do_something() {}
+}
+
+struct OverrideConst;
+impl Marker for OverrideConst { // error!
+    const N: usize = 1;
+}
+# fn main() {}
+```
+
+Because marker traits are allowed to have multiple implementations for the same
+type, it's not allowed to override anything in those implementations, as it
+would be ambiguous which override should actually be used.