type constraints from multiple || conditions don't propagate well

[aviatesk]

Originally posted at https://github.com/JuliaLang/julia/pull/39549/files#r571434978

Constraints don't propagate well in the following case:

julia> Base.return_types((Union{Int,Nothing},)) do x
           if x === nothing || x < 0
               return 0
           end
           x # x::Int, ideally
       end
1-element Vector{Any}:
 Union{Nothing, Int64}

Ideas to fix are quoted from @vtjnash 's response:

It is the lowering structure. For && it is lowered to a nested if block, but not so for ||. So for entry to #4, we have the tmerge for [x = Nothing, _3 = Conditional(:x, Nothing, Int)] and [x = Int, _3 = Bool]. Instead, lowering could fuse the goto #4 into the next line (jump-threading), and do goto #5 there to fix this. Or we should narrow the later _3 = Bool to the equivalent _3 = Conditional(x, Int, Int) before the corresponding tmerge. Either way, a good update to consider for later:

julia> code_typed((Union{Int,Nothing},), optimize=false) do x
           if x === nothing || x < 0
               return 0
           end
           x # x::Int, ideally
       end
1-element Vector{Any}:
 CodeInfo(
    @ REPL[6]:2 within `#13'
1 ─ %1 = (x === Main.nothing)::Bool
└──      goto #3 if not %1
2 ─      (@_3 = %1)::Bool
└──      goto #4
3 ─      (@_3 = x::Int64 < 0)::Bool
4 ┄      goto #6 if not @_3
    @ REPL[6]:3 within `#13'
5 ─      return 0
    @ REPL[6]:5 within `#13'
6 ─      return x
) => Union{Nothing, Int64}

[vtjnash]

We still might want to do the inference improvement too, since that should handle the case where the user wrote it out rather verbosely (like below). It might however be odd that Conditional would get a better tmerge result than Const(true) however, so also perhaps not.

julia> function f(x)
         x = x[]
         y = x === nothing
         if y
           z = y
         else
           z = x < 0
         end
         if z
           return 0
         end
         return x
       end