inference: allows conditional object to propagate constraint multiple…

… times (JuliaLang#39936)

Currently we always `widenconditional` conditional var state, which
makes us unable to propagate constraints from conditional object
multiple times:
```julia
@test Base.return_types((Union{Nothing,Int},)) do a
    b = a === nothing
    c = b ? 0 : a # c::Int
    d = !b ? a : 0 # d::Int ideally, but Union{Int,Nothing}
    c, d
 end == Any[Tuple{Int,Int}] # fail
```

This PR keeps conditional var state when the update is came from a
conditional branching, and allows a conditional object to propagate
constraint multiple times as far as the subject of condition doesn't
change. AFAIU this is safe because the update from conditional
branching doesn't change the condition itself.

base/compiler/abstractinterpretation.jl

@@ -1773,12 +1773,12 @@ function typeinf_local(interp::AbstractInterpreter, frame::InferenceState)
                     frame.src.ssavaluetypes[pc] = t
                     lhs = stmt.args[1]
                     if isa(lhs, Slot)
-                        changes = StateUpdate(lhs, VarState(t, false), changes)
+                        changes = StateUpdate(lhs, VarState(t, false), changes, false)
                     end
                 elseif hd === :method
                     fname = stmt.args[1]
                     if isa(fname, Slot)
-                        changes = StateUpdate(fname, VarState(Any, false), changes)
+                        changes = StateUpdate(fname, VarState(Any, false), changes, false)
                     end
                 elseif hd === :inbounds || hd === :meta || hd === :loopinfo || hd === :code_coverage_effect
                     # these do not generate code
@@ -1843,7 +1843,7 @@ end
 
 function conditional_changes(changes::VarTable, @nospecialize(typ), var::Slot)
     if typ ⊑ (changes[slot_id(var)]::VarState).typ
-        return StateUpdate(var, VarState(typ, false), changes)
+        return StateUpdate(var, VarState(typ, false), changes, true)
     end
     return changes
 end

base/compiler/typelattice.jl

@@ -87,6 +87,7 @@ struct StateUpdate
     var::Union{Slot,SSAValue}
     vtype::VarState
     state::VarTable
+    conditional::Bool
 end
 
 # Represent that the type estimate has been approximated, due to "causes"
@@ -321,16 +322,19 @@ function stupdate!(state::Nothing, changes::StateUpdate)
         changeid = slot_id(changes.var::Slot)
         newst[changeid] = changes.vtype
         # remove any Conditional for this Slot from the vtable
-        for i = 1:length(newst)
-            newtype = newst[i]
-            if isa(newtype, VarState)
-                newtypetyp = ignorelimited(newtype.typ)
-                if isa(newtypetyp, Conditional) && slot_id(newtypetyp.var) == changeid
-                    newtypetyp = widenconditional(newtypetyp)
-                    if newtype.typ isa LimitedAccuracy
-                        newtypetyp = LimitedAccuracy(newtypetyp, newtype.typ.causes)
+        # (unless this change is came from the conditional)
+        if !changes.conditional
+            for i = 1:length(newst)
+                newtype = newst[i]
+                if isa(newtype, VarState)
+                    newtypetyp = ignorelimited(newtype.typ)
+                    if isa(newtypetyp, Conditional) && slot_id(newtypetyp.var) == changeid
+                        newtypetyp = widenconditional(newtypetyp)
+                        if newtype.typ isa LimitedAccuracy
+                            newtypetyp = LimitedAccuracy(newtypetyp, newtype.typ.causes)
+                        end
+                        newst[i] = VarState(newtypetyp, newtype.undef)
                     end
-                    newst[i] = VarState(newtypetyp, newtype.undef)
                 end
             end
         end
@@ -352,7 +356,8 @@ function stupdate!(state::VarTable, changes::StateUpdate)
         end
         oldtype = state[i]
         # remove any Conditional for this Slot from the vtable
-        if isa(newtype, VarState)
+        # (unless this change is came from the conditional)
+        if !changes.conditional && isa(newtype, VarState)
             newtypetyp = ignorelimited(newtype.typ)
             if isa(newtypetyp, Conditional) && slot_id(newtypetyp.var) == changeid
                 newtypetyp = widenconditional(newtypetyp)
@@ -393,16 +398,19 @@ function stupdate1!(state::VarTable, change::StateUpdate)
     end
     changeid = slot_id(change.var::Slot)
     # remove any Conditional for this Slot from the catch block vtable
-    for i = 1:length(state)
-        oldtype = state[i]
-        if isa(oldtype, VarState)
-            oldtypetyp = ignorelimited(oldtype.typ)
-            if isa(oldtypetyp, Conditional) && slot_id(oldtypetyp.var) == changeid
-                oldtypetyp = widenconditional(oldtypetyp)
-                if oldtype.typ isa LimitedAccuracy
-                    oldtypetyp = LimitedAccuracy(oldtypetyp, oldtype.typ.causes)
+    # (unless this change is came from the conditional)
+    if !change.conditional
+        for i = 1:length(state)
+            oldtype = state[i]
+            if isa(oldtype, VarState)
+                oldtypetyp = ignorelimited(oldtype.typ)
+                if isa(oldtypetyp, Conditional) && slot_id(oldtypetyp.var) == changeid
+                    oldtypetyp = widenconditional(oldtypetyp)
+                    if oldtype.typ isa LimitedAccuracy
+                        oldtypetyp = LimitedAccuracy(oldtypetyp, oldtype.typ.causes)
+                    end
+                    state[i] = VarState(oldtypetyp, oldtype.undef)
                 end
-                state[i] = VarState(oldtypetyp, oldtype.undef)
             end
         end
     end

test/compiler/inference.jl

@@ -1795,6 +1795,31 @@ end
     end == Any[Union{Nothing,Expr}]
 end
 
+@testset "branching on conditional object" begin
+    # simple
+    @test Base.return_types((Union{Nothing,Int},)) do a
+        b = a === nothing
+        return b ? 0 : a # ::Int
+    end == Any[Int]
+
+    # can use multiple times (as far as the subject of condition hasn't changed)
+    @test Base.return_types((Union{Nothing,Int},)) do a
+        b = a === nothing
+        c = b ? 0 : a # c::Int
+        d = !b ? a : 0 # d::Int
+        return c, d # ::Tuple{Int,Int}
+    end == Any[Tuple{Int,Int}]
+
+    # shouldn't use the old constraint when the subject of condition has changed
+    @test Base.return_types((Union{Nothing,Int},)) do a
+        b = a === nothing
+        c = b ? 0 : a # c::Int
+        a = 0
+        d = b ? a : 1 # d::Int, not d::Union{Nothing,Int}
+        return c, d # ::Tuple{Int,Int}
+    end == Any[Tuple{Int,Int}]
+end
+
 function f25579(g)
     h = g[]
     t = (h === nothing)