Issue #17925 has been updated by kddnewton (Kevin Newton). This is interesting. It surfaces and incompatibility we have. I didn't realize Ruby didn't allow this, so YARP allows `case expression in 42; end` to parse as expected by the OP here. If `parse.y` can't support this, I will need to explicitly disallow this in YARP. ---------------------------------------- Bug #17925: Pattern matching syntax using semicolon one-line https://bugs.ruby-lang.org/issues/17925#change-104372 * Author: koic (Koichi ITO) * Status: Open * Priority: Normal * ruby -v: ruby 3.1.0dev (2021-05-28T16:34:27Z master e56ba6231f) [x86_64-darwin19] * Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN ---------------------------------------- ## Summary There are the following differences between `case ... when` and` case ... in`. Is this an expected behavior? ```console % ruby -v ruby 3.1.0dev (2021-05-28T16:34:27Z master e56ba6231f) [x86_64-darwin19] % ruby -ce 'case expression when 42; end' Syntax OK % ruby -ce 'case expression in 42; end' -e:1: warning: One-line pattern matching is experimental, and the behavior may change in future versions of Ruby! -e:1: syntax error, unexpected `end', expecting `when' case expression in 42; end ``` So, I have two concerns. - Since the pattern matching syntax is different from `case ... when`,　can't user write semicolon one-line `case ... in` in the same semicolon one-line as `case ... when`? - Does `case expression in 42; end` display an experimental warning of one-line pattern matching. Right? This is reproduced in Ruby 3.1.0-dev and Ruby 3.0.1. ## Additional Information NOTE 1: I understand that only syntax that doesn't use `case` and `end` is experimental one-line pattern matching syntax. ``` % ruby -ce 'expression in 42' -e:1: warning: One-line pattern matching is experimental, and the behavior may change in future versions of Ruby! Syntax OK ``` NOTE 2: The syntax is OK if a semicolon is used between `expression` and `in`. But `case ... when` is a valid syntax to omit. ``` % ruby -e ruby -ce 'case expression; in 42; end' Syntax OK ``` -- https://bugs.ruby-lang.org/