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diff --git a/compiler/rustc_error_codes/src/error_codes/E0735.md b/compiler/rustc_error_codes/src/error_codes/E0735.md
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+Type parameter defaults cannot use `Self` on structs, enums, or unions.
+
+Erroneous code example:
+
+```compile_fail,E0735
+struct Foo<X = Box<Self>> {
+    field1: Option<X>,
+    field2: Option<X>,
+}
+// error: type parameters cannot use `Self` in their defaults.
+```