FULL PRODUCT VERSION :
java version "1.8.0_20"
Java(TM) SE Runtime Environment (build 1.8.0_20-b26)
Java HotSpot(TM) 64-Bit Server VM (build 25.20-b23, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
OS X Version 10.9.5

A DESCRIPTION OF THE PROBLEM :
The compiler fails to compile a piece of valid Java code:

Give the following Stream

Stream<String> items = Arrays.asList("1","2","3","4","5").stream();

The following line fails to compile:

int res = items.reduce(0, (i,s) -> i + Integer.valueOf(s), (a,b) -> a + b);

Whereas the following succeed:

Integer res = items.reduce(0, (i,s) -> i + Integer.valueOf(s), (a,b) -> a + b);

The error the compiler generates when using the primitive integer version is the following:



REGRESSION.  Last worked in version 8u11

ADDITIONAL REGRESSION INFORMATION: 
java version "1.8.0_20"
Java(TM) SE Runtime Environment (build 1.8.0_20-b26)
Java HotSpot(TM) 64-Bit Server VM (build 25.20-b23, mixed mode)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Simply try to compile this:

Stream<String> items = Arrays.asList("1","2","3","4","5").stream();
int res = items.reduce(0, (i,s) -> i + Integer.valueOf(s), (a,b) -> a + b);

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
It should compile without problems.
ACTUAL -
An exception has occurred in the compiler (1.8.0_25).

ERROR MESSAGES/STACK TRACES THAT OCCUR :
An exception has occurred in the compiler (1.8.0_25). Please file a bug at the Java Developer Connection (http://java.sun.com/webapps/bugreport)  after checking the Bug Parade for duplicates. Include your program and the following diagnostic in your report.  Thank you.
java.lang.NullPointerException
	at com.sun.tools.javac.code.Types.isConvertible(Types.java:290)
	at com.sun.tools.javac.comp.Check.assertConvertible(Check.java:922)
	at com.sun.tools.javac.comp.Check.checkMethod(Check.java:876)
	at com.sun.tools.javac.comp.Attr.checkMethod(Attr.java:3838)
	at com.sun.tools.javac.comp.Attr.checkIdInternal(Attr.java:3615)
	at com.sun.tools.javac.comp.Attr.checkMethodIdInternal(Attr.java:3522)
	at com.sun.tools.javac.comp.Attr.checkMethodId(Attr.java:3501)
	at com.sun.tools.javac.comp.Attr.checkId(Attr.java:3488)
	at com.sun.tools.javac.comp.Attr.visitSelect(Attr.java:3370)
	at com.sun.tools.javac.tree.JCTree$JCFieldAccess.accept(JCTree.java:1897)
	at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:607)
	at com.sun.tools.javac.comp.Attr.visitApply(Attr.java:1843)
	at com.sun.tools.javac.tree.JCTree$JCMethodInvocation.accept(JCTree.java:1465)
	at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:607)
	at com.sun.tools.javac.comp.Attr.attribExpr(Attr.java:649)
	at com.sun.tools.javac.comp.Attr.visitVarDef(Attr.java:1093)
	at com.sun.tools.javac.tree.JCTree$JCVariableDecl.accept(JCTree.java:852)
	at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:607)
	at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:676)
	at com.sun.tools.javac.comp.Attr.attribStats(Attr.java:692)
	at com.sun.tools.javac.comp.Attr.visitBlock(Attr.java:1142)
	at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:909)
	at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:607)
	at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:676)
	at com.sun.tools.javac.comp.Attr.visitMethodDef(Attr.java:1035)
	at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:778)
	at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:607)
	at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:676)
	at com.sun.tools.javac.comp.Attr.attribClassBody(Attr.java:4342)
	at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4252)
	at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4181)
	at com.sun.tools.javac.comp.Attr.attrib(Attr.java:4156)
	at com.sun.tools.javac.main.JavaCompiler.attribute(JavaCompiler.java:1248)
	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 com.sun.tools.javac.Main.main(Main.java:42)


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
public static void main(String[] args) {

    Stream<String> items = Arrays.asList("1","2","3","4","5").stream();

    int res = items.reduce(0, (i,s) -> i + Integer.valueOf(s), (a,b) -> a + b);
    System.out.println(res);

  }
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
The workaround is to use wrapper objects.

Stream<String> items = Arrays.asList("1","2","3","4","5").stream();
Integer res = items.reduce(0, (i,s) -> i + Integer.valueOf(s), (a,b) -> a + b);