Ruby 2.6 and 2.7 have slightly different implementations of the String#@- method. In Ruby 2.6, the receiver of the String#@- method is modified under certain circumstances. This was later identified as a bug (https://bugs.ruby-lang.org/issues/15926) and only fixed in Ruby 2.7.

Before the changes in this commit, the ActiveRecord::ConnectionAdapters::SchemaCache#deep_deduplicate method, which internally calls the String#@- method, could also modify an input string argument in Ruby 2.6 -- changing a tainted, unfrozen string into a tainted, frozen string.