Major Description
The changes done in LOG4J2-1286 to deprecate MessageSupplier were not properly tested in practice. Using log4j-api ever since the deprecation, the following code causes deprecation warnings:
logger.info(() -> new SimpleMessage("Hello, world!"));
This is because the compiler interprets this as a MessageSupplier instead of the more generic Supplier<Message> version which is the intended API. Although manual use of MessageSupplier should be discouraged, users should not have to manually cast the above lambda just to prevent a deprecation warning:
logger.info((Supplier<Message>) () -> new SimpleMessage("Hello, world!"));
MessageSupplier should become a normal part of log4j-api with a note on both it and Supplier<T> that these interfaces would be removed in a log4j-api 3.0 release and replaced with the Java 8 version.