[ruby-core:106247] [Ruby master Bug#17865] clang 12 -Wcompound-token-split-by-macro warning in ruby.h

Issue #17865 has been updated by usa (Usaku NAKAMURA).

Backport changed from 2.6: REQUIRED, 2.7: REQUIRED, 3.0: DONTNEED to 2.6: REQUIRED, 2.7: DONE, 3.0: DONTNEED

backported into ruby_2_7 via PR. thx!

----------------------------------------
Bug #17865: clang 12 -Wcompound-token-split-by-macro warning in ruby.h
https://bugs.ruby-lang.org/issues/17865#change-94867

* Author: _dim (Dimitry Andric)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 2.7.3p183 (2021-04-05 revision 6847ee089d) [amd64-freebsd14]
* Backport: 2.6: REQUIRED, 2.7: DONE, 3.0: DONTNEED
----------------------------------------
As reported in [pull request \#4504](https://github.com/ruby/ruby/pull/4504) (originally via [FreeBSD PR 255910](https://bugs.freebsd.org/255910)), certain ruby gem native extensions (such as thrift, see [here](http://package22.nyi.freebsd.org/data/mainamd64PR255570-default/2021-05-08_16h02m24s/logs/errors/rubygem-thrift-0.14.0,1.log) and [here](http://package22.nyi.freebsd.org/data/mainamd64PR255570-default/2021-05-08_16h02m24s/logs/errors/unit-ruby2.7-1.23.0.log)), with clang 12.0.0 or later fails, because they have -Werror in their CFLAGS, resulting in complaints about the expansion of the `rb_intern()` macro:

```
current directory: /wrkdirs/usr/ports/devel/rubygem-thrift/work/stage/usr/local/lib/ruby/gems/2.7/gems/thrift-0.14.0/ext
make "DESTDIR="
compiling binary_protocol_accelerated.c
binary_protocol_accelerated.c:404:68: error: '(' and '{' tokens introducing statement expression appear in different macro expansion contexts [-Werror,-Wcompound-token-split-by-macro]
  VALUE thrift_binary_protocol_class = rb_const_get(thrift_module, rb_intern("BinaryProtocol"));
                                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/ruby-2.7/ruby/ruby.h:1847:23: note: expanded from macro 'rb_intern'
        __extension__ (RUBY_CONST_ID_CACHE((ID), (str))) : \
                      ^
binary_protocol_accelerated.c:404:68: note: '{' token is here
  VALUE thrift_binary_protocol_class = rb_const_get(thrift_module, rb_intern("BinaryProtocol"));
                                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/ruby-2.7/ruby/ruby.h:1847:24: note: expanded from macro 'rb_intern'
        __extension__ (RUBY_CONST_ID_CACHE((ID), (str))) : \
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/ruby-2.7/ruby/ruby.h:1832:5: note: expanded from macro 'RUBY_CONST_ID_CACHE'
    {                                                   \
    ^
```

Part of the `rb_intern()` macro expands to `(RUBY_CONST_ID_CACHE((ID), (str)))`, and in turn `RUBY_CONST_ID_CACHE()` expands to a brace enclosed compound statement. The intended effect is to get a gcc statement expression, which is normally delimited by `({ ... })`.

However, clang 12.0.0 and later have a warning enabled by default, about pasting together the `(` and `{` tokens via different macros (see [llvm-project@0e00a95](https://github.com/llvm/llvm-project/commit/0e00a95b4fad5e72851de012d3a0b2c2d01f8685)).

In the [pull request](https://github.com/ruby/ruby/pull/4504) I have submitted a possible fix for ruby 2.7 (and 2.6), because the above warnings do *not* occur on 3.0 and master: in [GitHub commit 6ecf07ab35](https://github.com/ruby/ruby/commit/6ecf07ab3538bfb74d298121ac0a1b2079b1d3fa) (for [pull request \#2991](https://github.com/ruby/ruby/pull/2991)) the `symbol.h` header got split off from the main `ruby.h`, and at the same time the code that emits the warning was removed. Later in commit commit:9e6e39c351 this got merged into master.




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