form conditional constraint from non-Conditional object

We can form a conditional constraint from a branch condition itself
when its type isn't already `Conditional` but can be refined as
`Conditional(condx, Const(true), Const(false))`.

base/compiler/abstractinterpretation.jl

@@ -1667,13 +1667,17 @@ function typeinf_local(interp::AbstractInterpreter, frame::InferenceState)
  elseif isa(stmt, GotoNode)
  pc´ = (stmt::GotoNode).label
  elseif isa(stmt, GotoIfNot)
- condt = abstract_eval_value(interp, stmt.cond, changes, frame)
+ condx = stmt.cond
+ condt = abstract_eval_value(interp, condx, changes, frame)
  if condt === Bottom
  empty!(frame.pclimitations)
- end
- if condt === Bottom
  break
  end
+ if !(isa(condt, Const) || isa(condt, Conditional)) && isa(condx, SlotNumber)
+ # if this non-`Conditional` object is a slot, we form and propagate
+ # the conditional constraint on it
+ condt = Conditional(condx, Const(true), Const(false))
+ end
  condval = maybe_extract_const_bool(condt)
  l = stmt.dest::Int
  if !isempty(frame.pclimitations)

test/compiler/inference.jl

@@ -1833,6 +1833,24 @@ end
  end == Any[Tuple{Int,Int}]
 end
 
+@testset "conditional constraint propagation from non-`Conditional` object" begin
+ @test Base.return_types((Bool,)) do b
+ if b
+ return !b ? nothing : 1 # ::Int
+ else
+ return 0
+ end
+ end == Any[Int]
+
+ @test Base.return_types((Any,)) do b
+ if b
+ return b # ::Bool
+ else
+ return nothing
+ end
+ end == Any[Union{Bool,Nothing}]
+end
+
 function f25579(g)
  h = g[]
  t = (h === nothing)