[runarberg]

Julia

  !(a::Int) = !bool(a)
  !0 + 2 + 2

  ##> 5

[orangecat]

Or more directly:

  julia> +(a::Int, b::Int) = if a==b==2 5 else a-(-b) end
  julia> 2+2
  5
  julia> 3+1
  4

(The else part can't use + or it will cause infinite recursion).