Fix a precision issue in abstract_iteration (#41839)

If the first loop exits in the first iteration, the `statetype` is still
`Bottom`. In that case, the new `stateordonet` needs to be determined
with the two-arg version of `iterate` again.

Explicitly test that inference produces a sound (and reasonably precise)
result when splatting an iterator (in this case a long range) that
allows constant-propagation up to the `MAX_TUPLE_SPLAT` limit.

Fixes #41022

Co-authored-by: Jameson Nash <vtjnash@gmail.com>

base/compiler/abstractinterpretation.jl

@@ -844,9 +844,11 @@ function abstract_iteration(interp::AbstractInterpreter, @nospecialize(itft), @n
             return ret, AbstractIterationInfo(calls)
         end
         if Nothing <: stateordonet_widened || length(ret) >= InferenceParams(interp).MAX_TUPLE_SPLAT
+            stateordonet = stateordonet_widened
             break
         end
         if !isa(stateordonet_widened, DataType) || !(stateordonet_widened <: Tuple) || isvatuple(stateordonet_widened) || length(stateordonet_widened.parameters) != 2
+            stateordonet = stateordonet_widened
             break
         end
         nstatetype = getfield_tfunc(stateordonet, Const(2))
@@ -864,27 +866,40 @@ function abstract_iteration(interp::AbstractInterpreter, @nospecialize(itft), @n
     end
     # From here on, we start asking for results on the widened types, rather than
     # the precise (potentially const) state type
-    statetype = widenconst(statetype)
-    valtype = widenconst(valtype)
+    # statetype and valtype are reinitialized in the first iteration below from the
+    # (widened) stateordonet, which has not yet been fully analyzed in the loop above
+    statetype = Bottom
+    valtype = Bottom
+    may_have_terminated = Nothing <: stateordonet
     while valtype !== Any
-        stateordonet = abstract_call_known(interp, iteratef, nothing, Any[Const(iteratef), itertype, statetype], sv).rt
-        stateordonet = widenconst(stateordonet)
-        nounion = typesubtract(stateordonet, Nothing, 0)
-        if !isa(nounion, DataType) || !(nounion <: Tuple) || isvatuple(nounion) || length(nounion.parameters) != 2
+        nounion = typeintersect(stateordonet, Tuple{Any,Any})
+        if nounion !== Union{} && !isa(nounion, DataType)
+            # nounion is of a type we cannot handle
             valtype = Any
             break
         end
-        if nounion.parameters[1] <: valtype && nounion.parameters[2] <: statetype
+        if nounion === Union{} || (nounion.parameters[1] <: valtype && nounion.parameters[2] <: statetype)
+            # reached a fixpoint or iterator failed/gave invalid answer
             if typeintersect(stateordonet, Nothing) === Union{}
-                # Reached a fixpoint, but Nothing is not possible => iterator is infinite or failing
-                return Any[Bottom], nothing
+                # ... but cannot terminate
+                if !may_have_terminated
+                    #  ... and cannot have terminated prior to this loop
+                    return Any[Bottom], nothing
+                else
+                    # iterator may have terminated prior to this loop, but not during it
+                    valtype = Bottom
+                end
             end
             break
         end
         valtype = tmerge(valtype, nounion.parameters[1])
         statetype = tmerge(statetype, nounion.parameters[2])
+        stateordonet = abstract_call_known(interp, iteratef, nothing, Any[Const(iteratef), itertype, statetype], sv).rt
+        stateordonet = widenconst(stateordonet)
+    end
+    if valtype !== Union{}
+        push!(ret, Vararg{valtype})
     end
-    push!(ret, Vararg{valtype})
     return ret, nothing
 end
 

test/compiler/inference.jl

@@ -2877,9 +2877,24 @@ partial_return_2(x) = Val{partial_return_1(x)[2]}
 
 @test Base.return_types(partial_return_2, (Int,)) == Any[Type{Val{1}}]
 
-# Precision of abstract_iteration
+# Soundness and precision of abstract_iteration
+f41839() = (1:100...,)
+@test NTuple{100,Int} <: only(Base.return_types(f41839, ())) <: Tuple{Vararg{Int}}
 f_splat(x) = (x...,)
 @test Base.return_types(f_splat, (Pair{Int,Int},)) == Any[Tuple{Int, Int}]
+@test Base.return_types(f_splat, (UnitRange{Int},)) == Any[Tuple{Vararg{Int}}]
+struct Itr41839_1 end # empty or infinite
+Base.iterate(::Itr41839_1) = rand(Bool) ? (nothing, nothing) : nothing
+Base.iterate(::Itr41839_1, ::Nothing) = (nothing, nothing)
+@test Base.return_types(f_splat, (Itr41839_1,)) == Any[Tuple{}]
+struct Itr41839_2 end # empty or failing
+Base.iterate(::Itr41839_2) = rand(Bool) ? (nothing, nothing) : nothing
+Base.iterate(::Itr41839_2, ::Nothing) = error()
+@test Base.return_types(f_splat, (Itr41839_2,)) == Any[Tuple{}]
+struct Itr41839_3 end
+Base.iterate(::Itr41839_3 ) = rand(Bool) ? nothing : (nothing, 1)
+Base.iterate(::Itr41839_3 , i) = i < 16 ? (i, i + 1) : nothing
+@test only(Base.return_types(f_splat, (Itr41839_3,))) <: Tuple{Vararg{Union{Nothing, Int}}}
 
 # issue #32699
 f32699(a) = (id = a[1],).id