Introduced in b75. Reproduced on both solaris-sparc and Windows XP.

javac fails if the current directory contains a child directory with a .java extension. It doesn't matter what is being compiled.

Example:

mkdir foo
cd foo
touch a.java
mkdir b.java

javac a.java

Output:

An exception has occurred in the compiler (1.6.0-beta2). Please file a bug at th
e Java Developer Connection (http://java.sun.com/webapps/bugreport) after check
ing the Bug Parade for duplicates. Include your program and the following diagno
stic in your report. Thank you.
java.lang.IllegalArgumentException: directories not supported
        at com.sun.tools.javac.util.DefaultFileManager$RegularFileObject.<init>(
DefaultFileManager.java:642)
        at com.sun.tools.javac.util.DefaultFileManager.list(DefaultFileManager.j
ava:340)
        at com.sun.tools.javac.util.DefaultFileManager.listAll(DefaultFileManage
r.java:292)
        at com.sun.tools.javac.util.DefaultFileManager.list(DefaultFileManager.j
ava:256)
        at com.sun.tools.javac.util.DefaultFileManager.list(DefaultFileManager.j
ava:54)
        at com.sun.tools.javac.jvm.ClassReader.fillIn(ClassReader.java:1951)
        at com.sun.tools.javac.jvm.ClassReader.complete(ClassReader.java:1703)
        at com.sun.tools.javac.code.Symbol.complete(Symbol.java:380)
        at com.sun.tools.javac.comp.Enter.visitTopLevel(Enter.java:258)
        at com.sun.tools.javac.tree.JCTree$JCCompilationUnit.accept(JCTree.java:
445)
        at com.sun.tools.javac.comp.Enter.classEnter(Enter.java:223)
        at com.sun.tools.javac.comp.Enter.classEnter(Enter.java:237)
        at com.sun.tools.javac.comp.Enter.complete(Enter.java:429)
        at com.sun.tools.javac.comp.Enter.main(Enter.java:414)
        at com.sun.tools.javac.main.JavaCompiler.enterTrees(JavaCompiler.java:71
9)
        at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:631)
        at com.sun.tools.javac.main.Main.compile(Main.java:766)
        at com.sun.tools.javac.main.Main.compile(Main.java:691)
        at com.sun.tools.javac.main.Main.compile(Main.java:680)
        at com.sun.tools.javac.Main.compile(Main.java:70)
        at com.sun.tools.javac.Main.main(Main.java:55)