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diff --git a/compiler/rustc_error_codes/src/error_codes/E0520.md b/compiler/rustc_error_codes/src/error_codes/E0520.md
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+A non-default implementation was already made on this type so it cannot be
+specialized further.
+
+Erroneous code example:
+
+```compile_fail,E0520
+#![feature(specialization)]
+
+trait SpaceLlama {
+    fn fly(&self);
+}
+
+// applies to all T
+impl<T> SpaceLlama for T {
+    default fn fly(&self) {}
+}
+
+// non-default impl
+// applies to all `Clone` T and overrides the previous impl
+impl<T: Clone> SpaceLlama for T {
+    fn fly(&self) {}
+}
+
+// since `i32` is clone, this conflicts with the previous implementation
+impl SpaceLlama for i32 {
+    default fn fly(&self) {}
+    // error: item `fly` is provided by an `impl` that specializes
+    //        another, but the item in the parent `impl` is not marked
+    //        `default` and so it cannot be specialized.
+}
+```
+
+Specialization only allows you to override `default` functions in
+implementations.
+
+To fix this error, you need to mark all the parent implementations as default.
+Example:
+
+```
+#![feature(specialization)]
+
+trait SpaceLlama {
+    fn fly(&self);
+}
+
+// applies to all T
+impl<T> SpaceLlama for T {
+    default fn fly(&self) {} // This is a parent implementation.
+}
+
+// applies to all `Clone` T; overrides the previous impl
+impl<T: Clone> SpaceLlama for T {
+    default fn fly(&self) {} // This is a parent implementation but was
+                             // previously not a default one, causing the error
+}
+
+// applies to i32, overrides the previous two impls
+impl SpaceLlama for i32 {
+    fn fly(&self) {} // And now that's ok!
+}
+```