The place to look is JLS ยง15.27.1. Lambda Parameters
It is a compile-time error if a lambda parameter has the name _ (that is, a single underscore character).
The use of the variable name _ in any context is discouraged. Future versions of the Java programming language may reserve this name as a keyword and/or give it special semantics.
So the Eclipse message is misleading, especially as the same message is used for both cases, when an error is generated for a lambda parameter or when a warning is generated for any other _ identifier.