_ (underscore) is a reserved keyword

by

The place to look is JLS §15.27.1. 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.

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