Ruby

The following syntax behavior differs between Ruby 3.3 (parse.y by default) and Ruby 3.4 (Prism by default).

if true && not true; end 

Expected¶

In Ruby 3.3 and earlier, the following code results in a syntax error. The default parser is parse.y.

$ ruby -vce 'if true && not true; end' ruby 3.3.8 (2025-04-09 revision b200bad6cd) [x86_64-darwin24] -e:1: syntax error, unexpected `true', expecting '(' if true && not true; end ruby: compile error (SyntaxError) 

The key point is that not is used on the right-hand side of a logical operator.

Actual¶

When the default parser is switched to Prism, it is accepted as valid syntax with Ruby 3.4+ (Prism by default):

$ ruby -vce 'if true && not true; end' ruby 3.4.3 (2025-04-14 revision d0b7e5b6a0) +PRISM [x86_64-darwin24] Syntax OK $ ruby -vce 'if true && not true; end' ruby 3.5.0dev (2025-05-13T02:05:19Z master 9b8c846bdf) +PRISM [x86_64-darwin24] Syntax OK 

Specifying parse.y causes a syntax error, as before 3.3 (parse.y by default):

$ ruby -vce 'if true && not true; end' ruby 3.3.8 (2025-04-09 revision b200bad6cd) [x86_64-darwin24] -e:1: syntax error, unexpected `true', expecting '(' if true && not true; end ruby: compile error (SyntaxError) $ ruby --parser=parse.y -vce 'if true && not true; end' ruby 3.5.0dev (2025-05-13T02:05:19Z master 9b8c846bdf) [x86_64-darwin24] -e:1: syntax error, unexpected 'true', expecting '(' if true && not true; end ruby: compile error (SyntaxError) 

The behavior seen in the parse.y producing a syntax error might be probably the expected one.
Any case, it is likely that users would not expect such a discrepancy between parsers.

This issue was noticed in the context of the following RuboCop issue:
https://github.com/rubocop/rubocop/issues/14177