Performance/Sum is not type-safe and may introduce a bug

[drbrain]

Enumerable#reduce and Array#sum have different behavior on an empty list:

$ irb
>> [].reduce(&:+)
=> nil
>> [].sum
=> 0

If code is dependent upon this behavior through using nil to indicate that no values were in the list, applying this change may introduce bugs.

Expected behavior

Rubocop only suggests type-safe changes or appropriately warns about type-safety risks in its suggestions.

Actual behavior

Rubocop equates reduce(&:+) and sum as if they are completely equivalent.

Steps to reproduce the problem

See irb output above

You can also run rubocop -r rubocop-performance --only Performance/Sum on this file:

def m
  [].reduce(&:+)
end

It will say:

t.rb:2:6: C: Performance/Sum: Use sum instead of reduce(&:+).

But nothing about type safety.

RuboCop version

Include the output of rubocop -V or bundle exec rubocop -V if using Bundler. Here's an example:

0.92.0 (using Parser 2.7.1.5, rubocop-ast 0.7.1, running on ruby 2.6.6 x86_64-darwin19)

Also, rubocop-performance 1.8.1