[ruby-core:109078] [Ruby master Bug#18884] parse.y: inconsistent parsing of "if" after "class"

Issue #18884 has been reported by qnighy (Masaki Hara).

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Bug #18884: parse.y: inconsistent parsing of "if" after "class"
https://bugs.ruby-lang.org/issues/18884

* Author: qnighy (Masaki Hara)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-linux]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
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There is a corner case where `if` inconsistently causes a syntax error:

```ruby
#  syntax error, unexpected `if' modifier
class if 1; Object end::Object
end
```

compare the following:

```ruby
# Parses
module if 1; Object end::Kernel
end
```

We use `EXPR_CLASS` after the class keyword to temporarily disable heredocs as in `class <<self; end`. This state is expected to behave like EXPR_BEG in other cases but the above case fails to take it into account.



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