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+A negative implementation was marked as unsafe.
+
+Erroneous code example:
+
+```compile_fail,E0198
+struct Foo;
+
+unsafe impl !Clone for Foo { } // error!
+```
+
+A negative implementation is one that excludes a type from implementing a
+particular trait. Not being able to use a trait is always a safe operation,
+so negative implementations are always safe and never need to be marked as
+unsafe.
+
+This will compile:
+
+```ignore (ignore auto_trait future compatibility warning)
+#![feature(auto_traits)]
+
+struct Foo;
+
+auto trait Enterprise {}
+
+impl !Enterprise for Foo { }
+```
+
+Please note that negative impls are only allowed for auto traits.