FULL PRODUCT VERSION :
$ java -version
java version "1.6.0_0"
OpenJDK Runtime Environment (IcedTea6 1.6) (fedora-33.b16.fc12-x86_64)
OpenJDK 64-Bit Server VM (build 14.0-b16, mixed mode)

Also latest jdk 1.6 on windows, and jdk 1.7 prerelease.

ADDITIONAL OS VERSION INFORMATION :
(irrelevant).

A DESCRIPTION OF THE PROBLEM :
Attached code contains two loops; one an enhanced for-loop, the other the for-loop manually desugared as per JLS3 14.14.2.

They should behave identically. They do not. The manually desugared loop generates a class-cast exception, while the enhanced for-loop does not.

Inspecting the byte-code output from the compiler, the result of .iterator() is not-being run-time type-checked before being stored to the synthesized variable. Such a check is necessary when the type after erasure differs from the unerased type: the desugar from the JLS is:

for (I #i = Expression.iterator(); #i.hasNext(); ) {

        VariableModifiersopt Type Identifier = #i.next();
   Statement
}

In the example below, Expression.iterator() has type MyIterator, but after erasure the type is Iterator, and thus a run-time type check is necessary.

Plausibly, this could be resolved by changing the JLS, rather than javac, by specifying the type of #i to be Iterator<Type>, where Type is as per 14.14.2.


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------

import java.util.ArrayList;
import java.util.Iterator;

class MyIterable<T extends Iterator<String> > implements Iterable<String> {
    public T iterator() { return stored; }
    public T stored;
}

class MyIterator implements Iterator<String> {

    public boolean hasNext() { return stored.hasNext(); }
    public String next() { return stored.next(); }
    public void remove() { }

    public Iterator<String> stored;
}

public class Temp
{
    public static void main(String[] args) {

        MyIterable<MyIterator> u = new MyIterable<MyIterator>();

        MyIterable unsafe = u;
        unsafe.stored = new ArrayList<String>().iterator();

        System.out.println ("Sugared....");
        for (String s : u)
            System.out.println (s);

        System.out.println ("Unsugared....");
        for (MyIterator i = u.iterator(); i.hasNext(); ) {
            String s = i.next();
            System.out.println (s);
        }
    }
}

---------- END SOURCE ----------