FULL PRODUCT VERSION :
java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.121-b13, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Linux pamela 4.4.0-59-lowlatency #80-Ubuntu SMP PREEMPT Fri Jan 6 21:04:05 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux


A DESCRIPTION OF THE PROBLEM :
Impossible to compile the attached code => the error is
RunTransformation.java:41: error: method unmodifiableSet in class Collections cannot be applied to given types;
		final Set<String> transformerCodes = Collections.unmodifiableSet(mediaProcessAbstractFactories.stream()
		                                                ^
  required: Set<? extends T>
  found: Collection<String>
  reason: cannot infer type-variable(s) E
    (actual and formal argument lists differ in length)
  where T,E are type-variables:
    T extends Object declared in method <T>unmodifiableSet(Set<? extends T>)
    E extends Object declared in class TreeSet


REGRESSION.  Last worked in version 8u102

ADDITIONAL REGRESSION INFORMATION: 
java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
javac RunTransformation.java

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
RunTransformation.class and RunTransformation$MyClass.class created
ACTUAL -
no compiling

ERROR MESSAGES/STACK TRACES THAT OCCUR :
error: method unmodifiableSet in class Collections cannot be applied to given types;
		final Set<String> transformerCodes = Collections.unmodifiableSet(mediaProcessAbstractFactories.stream()
		                                                ^
  required: Set<? extends T>
  found: Collection<String>
  reason: cannot infer type-variable(s) E
    (actual and formal argument lists differ in length)
  where T,E are type-variables:
    T extends Object declared in method <T>unmodifiableSet(Set<? extends T>)
    E extends Object declared in class TreeSet


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.util.*;

import java.util.stream.Collectors;

/**
 * @author Marian
 *
 */
public final class RunTransformation {
	public static void main(final String... args) {
		List<MyClass> mediaProcessAbstractFactories = createMediaProcessAbstractFactoryList ();
		final Set<String> transformerCodes = Collections.unmodifiableSet(mediaProcessAbstractFactories.stream()
                .flatMap(mpaf -> mpaf.getCodes().stream())
                .collect(Collectors.toCollection(TreeSet::new)));
		System.out.println ("transformerCodes size = " + transformerCodes.size());
	}

	private static List<MyClass> createMediaProcessAbstractFactoryList() {
		return Arrays.asList(new MyClass (1), new MyClass (2));
	}

	private static final class MyClass {
		private final Set<String> codes = new HashSet<>();
		MyClass (int nbc) {
			for (int i = 0; i < nbc; i++) {
				codes.add(String.valueOf(i));
			}
		}
	
		Set<String> getCodes() {
			return codes;
		}
		
	}
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
cast:
final Set<String> transformerCodes = Collections.unmodifiableSet((Set<String>)mediaProcessAbstractFactories.stream()
                .flatMap(mpaf -> mpaf.getCodes().stream())
                .collect(Collectors.toCollection(TreeSet::new)));