Issue #16480 has been updated by shyouhei (Shyouhei Urabe).

Assignee set to nobu (Nobuyoshi Nakada)
Status changed from Open to Assigned

It indeed seems to be a bug.  For C extensions, that arity argument can be a variable.  There is a fallback code.  But for C++, there is no way to accept non-constexpr arities.  This is a problem.

@nobu Can you provide us a workaround?  Or revert, because there is no way to support the extension library?

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Bug #16480: Ruby 2.7, C++, and rb_define_method compilation error
https://bugs.ruby-lang.org/issues/16480#change-83660

* Author: jasonroelofs (Jason Roelofs)
* Status: Assigned
* Priority: Normal
* Assignee: nobu (Nobuyoshi Nakada)
* Target version: 
* ruby -v: ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-darwin18]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
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I maintain the Rice library which provides constructs for exposing C++ libraries into Ruby. With Ruby 2.7.0 I've got a hard compilation failure that I'm not sure how to fix.[1]

Diving into this, I've found that rb_define_method, when compiled under C++, is now a macro that uses templates to choose the correct overload by the given arity. However, this seems to require a constexpr-compatible use of rb_define_method, making the API unavailable for more dynamic uses[1].

The PR that made these changes is here[3].

I've looked for ways to get around this macro change but have been unable to make anything work. Is there a way I've missed to directly call the underlying C API function or is this now simply not possible with Ruby 2.7.0?

[1] https://github.com/jasonroelofs/rice/issues/119
[2] https://github.com/jasonroelofs/rice/blob/master/rice/detail/method_data.cpp#L78
[3] https://github.com/ruby/ruby/pull/2461




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