RFR: 8276789: Support C++ lambda in ResourceHashtable::iterate [v4]

[Kim Barrett]

On Wed, 25 May 2022 06:28:19 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> This is still needed as there are many places that use the old style iteration. Also, when the body of the iteration is complex, it might be more readable to do it as a separate class. E.g., here in systemDictionaryShared.cpp:
>> 
>> 
>>     UnregisteredClassesDuplicationChecker dup_checker;
>>     _dumptime_table->iterate(&dup_checker);
>> 
>> 
>> I think there's no ambiguity because `iter->do_entry()` cannot be substituted with the other template, and `function(node->_key, node->_value)` in the other template cannot be substituted here. So by SFINAE the C++ compiler can pick the correct template.
>
> Interesting. I got problems when I converted BitMap to support both closures and lambdas. The problem might have been that the iterate function took an specific closure class, and didn't use a template.

[Not a review, just a drive-by comment]
SFINAE operates on arguments and signatures; it doesn't reach into the bodies
of (potential) callees like that. The disambiguation is happening because one
argument (the Closure object) is passed as a pointer, while the other is a
lambda object (not a pointer). In the pointer to closure object case the ITER*
overload is a better match, so is selected. In the lambda case, it's not a
pointer so only the non-pointer overload is applicable.

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PR: https://git.openjdk.java.net/jdk/pull/8761