Invalid @UnnecessaryLambda patch suggestion

[Stephan202]

Consider the following code:

import java.util.stream.IntStream;

class Example {
  void someLoopyCode() {
    for (int i : someIterable()) {
      // Do something.
    }
  }

  private Iterable<Integer> someIterable() {
    return () -> IntStream.range(0, 42).boxed().iterator();
  }
}

Compile the code as follows:

javac \
  -XDcompilePolicy=simple \
  -processorpath error_prone_core-2.9.0-with-dependencies.jar \
  '-Xplugin:ErrorProne -XepDisableAllChecks -Xep:UnnecessaryLambda:WARN' \
  Example.java

This yields the following warning:

Example.java:10: warning: [UnnecessaryLambda] Returning a lambda from a helper method or saving it in a constant is unnecessary; prefer to implement the functional interface method directly and use a method reference instead.
  private Iterable<Integer> someIterable() {
                            ^
    (see https://errorprone.info/bugpattern/UnnecessaryLambda)
  Did you mean 'for (int i : this::someIterable) {'?
1 warning

If this suggestion were accepted using -XepPatchChecks:UnnecessaryLambda the resulting code would not compile, producing a "method reference not expected here" error. Possible solutions:

1. Don't flag enhanced for-loop method calls.
2. Introduce an explicit cast (i.e., suggest for (int i : (Iterable<Integer>) this::someIterable) { instead).

[cushon]

93ca62e