FULL PRODUCT VERSION :
 Java; VM; Vendor        = 1.8.0_71; Java HotSpot(TM) 64-Bit Server VM 25.71-b15; Oracle Corporation


ADDITIONAL OS VERSION INFORMATION :
  Operating System        = Windows 10 version 10.0 running on amd64


EXTRA RELEVANT SYSTEM CONFIGURATION :
>Log Session: Sunday, February 21, 2016 11:43:19 AM CET
>System Info: 
  Product Version         = NetBeans IDE 8.1 (Build 201510222201) (#bb5d68d8a020)
  Operating System        = Windows 10 version 10.0 running on amd64
  Java; VM; Vendor        = 1.8.0_71; Java HotSpot(TM) 64-Bit Server VM 25.71-b15; Oracle Corporation
  Runtime                 = Java(TM) SE Runtime Environment 1.8.0_71-b15


A DESCRIPTION OF THE PROBLEM :
When compiler worked with twou source trees, the error appeared. When i have removed one of them, the problem disappeared.

Other my projects have no problems with more source trees, I use it often.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Try to compile project EXPERIMENTS_Java. 
It uses the CanvasManager project as a library.

When you remove the src folder from the list of source folders, problem disappear.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I expected that the project will be compiled. Exception appeared instead.
ACTUAL -
Exception was thrown.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
An exception has occurred in the compiler (1.8.0_71). Please file a bug at the Java Bug Database (http://bugreport.java.com/bugreport/) after checking the database for duplicates. Include your program and the following diagnostic in your report.  Thank you.
java.lang.AssertionError
	at com.sun.tools.javac.util.Assert.error(Assert.java:126)
	at com.sun.tools.javac.util.Assert.check(Assert.java:45)
	at com.sun.tools.javac.code.Types.functionalInterfaceBridges(Types.java:659)
	at com.sun.tools.javac.comp.LambdaToMethod$LambdaAnalyzerPreprocessor$TranslationContext.<init>(LambdaToMethod.java:1692)
	at com.sun.tools.javac.comp.LambdaToMethod$LambdaAnalyzerPreprocessor$LambdaTranslationContext.<init>(LambdaToMethod.java:1775)
	at com.sun.tools.javac.comp.LambdaToMethod$LambdaAnalyzerPreprocessor.analyzeLambda(LambdaToMethod.java:1275)
	at com.sun.tools.javac.comp.LambdaToMethod$LambdaAnalyzerPreprocessor.visitLambda(LambdaToMethod.java:1260)
	at com.sun.tools.javac.tree.JCTree$JCLambda.accept(JCTree.java:1624)
	at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
	at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:70)
	at com.sun.tools.javac.tree.TreeTranslator.visitApply(TreeTranslator.java:275)
	at com.sun.tools.javac.tree.JCTree$JCMethodInvocation.accept(JCTree.java:1465)
	at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
	at com.sun.tools.javac.tree.TreeTranslator.visitVarDef(TreeTranslator.java:153)
	at com.sun.tools.javac.comp.LambdaToMethod$LambdaAnalyzerPreprocessor.visitVarDef(LambdaToMethod.java:1441)
	at com.sun.tools.javac.tree.JCTree$JCVariableDecl.accept(JCTree.java:852)
	at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
	at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:70)
	at com.sun.tools.javac.tree.TreeTranslator.visitBlock(TreeTranslator.java:162)
	at com.sun.tools.javac.comp.LambdaToMethod$LambdaAnalyzerPreprocessor.visitBlock(LambdaToMethod.java:1172)
	at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:909)
	at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
	at com.sun.tools.javac.tree.TreeTranslator.visitMethodDef(TreeTranslator.java:145)
	at com.sun.tools.javac.comp.LambdaToMethod$LambdaAnalyzerPreprocessor.visitMethodDef(LambdaToMethod.java:1299)
	at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:778)
	at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
	at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:70)
	at com.sun.tools.javac.tree.TreeTranslator.visitClassDef(TreeTranslator.java:134)
	at com.sun.tools.javac.comp.LambdaToMethod$LambdaAnalyzerPreprocessor.visitClassDef(LambdaToMethod.java:1209)
	at com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:693)
	at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
	at com.sun.tools.javac.comp.LambdaToMethod$LambdaAnalyzerPreprocessor.analyzeAndPreprocessClass(LambdaToMethod.java:1162)
	at com.sun.tools.javac.comp.LambdaToMethod$LambdaAnalyzerPreprocessor.access$300(LambdaToMethod.java:1119)
	at com.sun.tools.javac.comp.LambdaToMethod.visitClassDef(LambdaToMethod.java:232)
	at com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:693)
	at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
	at com.sun.tools.javac.comp.LambdaToMethod.translate(LambdaToMethod.java:197)
	at com.sun.tools.javac.comp.LambdaToMethod.translate(LambdaToMethod.java:190)
	at com.sun.tools.javac.comp.LambdaToMethod.translateTopLevelClass(LambdaToMethod.java:217)
	at com.sun.tools.javac.main.JavaCompiler.desugar(JavaCompiler.java:1493)
	at com.sun.tools.javac.main.JavaCompiler.desugar(JavaCompiler.java:1356)
	at com.sun.tools.javac.main.JavaCompiler.compile2(JavaCompiler.java:901)
	at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:860)
	at com.sun.tools.javac.main.Main.compile(Main.java:523)
	at com.sun.tools.javac.main.Main.compile(Main.java:381)
	at com.sun.tools.javac.main.Main.compile(Main.java:370)
	at com.sun.tools.javac.main.Main.compile(Main.java:361)
	at com.sun.tools.javac.Main.compile(Main.java:56)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:497)
	at org.apache.tools.ant.taskdefs.compilers.Javac13.execute(Javac13.java:56)
	at org.apache.tools.ant.taskdefs.Javac.compile(Javac.java:1159)
	at org.apache.tools.ant.taskdefs.Javac.execute(Javac.java:935)
	at org.netbeans.modules.java.source.ant.JavacTask.execute(JavacTask.java:145)
	at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:292)
	at sun.reflect.GeneratedMethodAccessor172.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:497)
	at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
	at org.apache.tools.ant.Task.perform(Task.java:348)
	at org.apache.tools.ant.taskdefs.Sequential.execute(Sequential.java:68)
	at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:292)
	at sun.reflect.GeneratedMethodAccessor172.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:497)
	at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
	at org.apache.tools.ant.Task.perform(Task.java:348)
	at org.apache.tools.ant.taskdefs.MacroInstance.execute(MacroInstance.java:396)
	at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:292)
	at sun.reflect.GeneratedMethodAccessor172.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:497)
	at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
	at org.apache.tools.ant.Task.perform(Task.java:348)
	at org.apache.tools.ant.Target.execute(Target.java:435)
	at org.apache.tools.ant.Target.performTasks(Target.java:456)
	at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1393)
	at org.apache.tools.ant.Project.executeTarget(Project.java:1364)
	at org.apache.tools.ant.helper.DefaultExecutor.executeTargets(DefaultExecutor.java:41)
	at org.apache.tools.ant.Project.executeTargets(Project.java:1248)
	at org.apache.tools.ant.module.bridge.impl.BridgeImpl.run(BridgeImpl.java:286)
	at org.apache.tools.ant.module.run.TargetExecutor.run(TargetExecutor.java:555)
	at org.netbeans.core.execution.RunClassThread.run(RunClassThread.java:153)
D:\Tvorba_PGM\# Projects_NetBeans\EXPERIMENTS_Java\nbproject\build-impl.xml:931: The following error occurred while executing this line:
D:\Tvorba_PGM\# Projects_NetBeans\EXPERIMENTS_Java\nbproject\build-impl.xml:271: Compile failed; see the compiler error output for details.
BUILD FAILED (total time: 0 seconds)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
I do not know, where the exception appeard. As I have said, when I removed the sourcecode tree src, the error disappeared. I can only sent the problematic projects. They are school projects and therefore they are relatively small (88+360 kB)
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
As I have said, when I removed the sourcecode tree src, the error disappeared.