Description:
------------
I originally reported this under #65317 but after looking into it a bit more 
(thanks laruence) it turned out that the original report was a little bit 
misleading:
the autoloader isn't prevented because we are already in an autoload call, but 
because the engine won't call the autoloaders for errors from compilation 
time(zend_lookup_class_ex):

    /* The compiler is not-reentrant. Make sure we __autoload() only during run-
time
     * (doesn't impact fuctionality of __autoload()
    */
    if (!use_autoload || zend_is_compiling(TSRMLS_C)) {
        if (!key) {
            free_alloca(lc_free, use_heap);
        }
        return FAILURE;
    }

I would be curious how come that it is safe to call the autoloader (which in 
turn can do anything, include other files or call the autoloaders manually), but 
not safe to call the autoloader in this case.

Test script:
---------------
<?php
set_error_handler(function($errno, $errstr, $errfile, $errline){
    	echo $errstr."\n";
        new MyClass;
});

spl_autoload_register(function($class){
    echo $class."\n";
    return eval("class $class{}");
});

eval('
	class MyConcrete extends MyAbstract {
		public static function createInstance() {}
	}
	abstract class MyAbstract {
		public abstract static function createInstance();
	}
');

Expected result:
----------------
call the autoloader

Actual result:
--------------
autoloader not called, fatal error is triggered by the missing class