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diff --git a/compiler/rustc_error_codes/src/error_codes/E0128.md b/compiler/rustc_error_codes/src/error_codes/E0128.md
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+A type parameter with default value is using forward declared identifier.
+
+Erroneous code example:
+
+```compile_fail,E0128
+struct Foo<T = U, U = ()> {
+    field1: T,
+    field2: U,
+}
+// error: generic parameters with a default cannot use forward declared
+//        identifiers
+```
+
+Type parameter defaults can only use parameters that occur before them. Since
+type parameters are evaluated in-order, this issue could be fixed by doing:
+
+```
+struct Foo<U = (), T = U> {
+    field1: T,
+    field2: U,
+}
+```
+
+Please also verify that this wasn't because of a name-clash and rename the type
+parameter if so.