Support endless ranges as the final argument to a when clause.

Author: kddnewton

CHANGELOG.md

@@ -10,6 +10,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/) a
 
 - Support for Ruby 3.1 syntax, including: blocks without names, hash keys without values, endless methods without parentheses, and new argument forwarding.
 - Support for pinned expressions and variables within pattern matching.
+- Support endless ranges as the final argument to a `when` clause.
 
 ## [1.1.1] - 2021-12-09
 

lib/syntax_tree.rb

@@ -13246,6 +13246,14 @@ def format(q)
               separator = -> { q.group { q.comma_breakable } }
               q.seplist(arguments.parts, separator) { |part| q.format(part) }
             end
+
+            # Very special case here. If you're inside of a when clause and the
+            # last argument to the predicate is and endless range, then you are
+            # forced to use the "then" keyword to make it parse properly.
+            last = arguments.parts.last
+            if (last.is_a?(Dot2) || last.is_a?(Dot3)) && !last.right
+              q.text(" then")
+            end
           end
         end
 

test/fixtures/when.rb

@@ -46,3 +46,11 @@
 when foo
 else
 end
+%
+case
+when foo.. then
+end
+%
+case
+when foo... then
+end