Bug #18878: parse.y: Foo::Bar {} is inconsistently rejected

• Status: Assigned
• Assignee: ydah (Yudai Takada)
• Target version: -
• ruby -v: ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-linux]
• Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN

[qnighy (Masaki Hara)]

The following source doesn't parse:

Foo::Bar {}

despite the following:

bar           # local variable or method call
Bar           # constant
bar()         # method call
Bar()         # method call
bar {}        # method call
Bar {}        # method call
bar() {}      # method call
Bar() {}      # method call
Foo::bar      # method call
Foo::Bar      # constant
Foo::bar()    # method call
Foo::Bar()    # method call
Foo::bar {}   # method call
# Foo::Bar {} # SyntaxError
Foo::bar() {} # method call
Foo::Bar() {} # method call

Especially considering Bar {}, this looks like a missing implementation rather than an intentional behavior.

[lhmzhou (Linda Zhou)]

Is there a more descriptive error message, or is SyntaxError the only output provided?

[qnighy (Masaki Hara)]

Is there a more descriptive error message, or is SyntaxError the only output provided?

This is the actual error message:

% ruby -e 'Foo::Bar {}'
-e:1: syntax error, unexpected '{'
Foo::Bar {}

[jeremyevans0 (Jeremy Evans)]

It looks like this will be fixed by YARP:

$ ruby -r yarp -e "pp YARP.parse('Foo::Bar {}')"
#<YARP::ParseResult:0x00000b1512acc448
 @comments=[],
 @errors=[],
 @source=#<YARP::Source:0x00000b14749d6690 @offsets=[0], @source="Foo::Bar {}">,
 @value=
  ProgramNode(0...11)(
    [],
    StatementsNode(0...11)(
      [CallNode(0...11)(ConstantReadNode(0...3)(), (3...5), (5...8), nil, nil, nil, BlockNode(9...11)([], nil, nil, (9...10), (10...11)), 0, "Bar")]
    )
  ),
 @warnings=[]>

[qnighy (Masaki Hara)]

Now Foo::Bar {} parses in parse.y too, but Foo::Bar {} + 1 still doesn't parse unlike in Prism.

[mame (Yusuke Endoh)]

• Status changed from Open to Assigned
• Assignee set to ydah (Yudai Takada)

[yui-knk (Kaneko Yuichiro)]

Do we also support Foo::Bar do end (and Foo::Bar do end + 1)?
With ruby 3.5.0dev (2025-08-27T03:02:56Z master 5ff7b2c582) both parse.y and prism have same behavior for do block cases like below.

bar do end        # method call
Bar do end        # method call
bar() do end      # method call
Bar() do end      # method call
Foo::bar do end   # method call
# Foo::Bar do end   # SyntaxError
Foo::bar() do end # method call
Foo::Bar() do end # method call

[yui-knk (Kaneko Yuichiro)]

Patch:

• Support only Foo::Bar {} + 1: https://github.com/yui-knk/ruby/tree/bug_18878_2
• Support both Foo::Bar {} + 1 and Foo::Bar do end (and Foo::Bar do end + 1): https://github.com/yui-knk/ruby/tree/bug_18878

[yui-knk (Kaneko Yuichiro)]

Note: 9930363aab6ac4b8d7034baff85cd86c17953dc9 was the commit to allow Foo::Bar {} in parse.y.