make Tuple{Union{}} unconstructable (#49111)

Type intersection assumed it was equal to Union{}, so this makes it
unconstructable so that holds true. This is similar to what the
NamedTuple constructor does.

Secondarily, this fixes an inference bug where it would create
Vararg{Union{}} and then incorrectly handle that fieldtype.

- Fixes #32392
- Addresses part of the concerns discussed in
  #24614 (comment)
- Addresses part of the issues presented in
  #26175
- May allow improving jl_type_equality_is_identity
  (https://github.com/JuliaLang/julia/pull/49017/files#diff-882927c6e612596e22406ae0d06adcee88a9ec05e8b61ad81b48942e2cb266e9R986)
- May allow improving intersection (finish_unionall can be more
  aggressive at computing varval for any typevars that appears in
  covariant position and has lb=Union{} and ub=leaf type)

Author: vtjnash

base/broadcast.jl

@@ -732,17 +732,21 @@ broadcastable(x) = collect(x)
 broadcastable(::Union{AbstractDict, NamedTuple}) = throw(ArgumentError("broadcasting over dictionaries and `NamedTuple`s is reserved"))
 
 ## Computation of inferred result type, for empty and concretely inferred cases only
-_broadcast_getindex_eltype(bc::Broadcasted) = Base._return_type(bc.f, eltypes(bc.args))
+_broadcast_getindex_eltype(bc::Broadcasted) = combine_eltypes(bc.f, bc.args)
 _broadcast_getindex_eltype(A) = eltype(A)  # Tuple, Array, etc.
 
 eltypes(::Tuple{}) = Tuple{}
-eltypes(t::Tuple{Any}) = Tuple{_broadcast_getindex_eltype(t[1])}
-eltypes(t::Tuple{Any,Any}) = Tuple{_broadcast_getindex_eltype(t[1]), _broadcast_getindex_eltype(t[2])}
-eltypes(t::Tuple) = Tuple{_broadcast_getindex_eltype(t[1]), eltypes(tail(t)).types...}
+eltypes(t::Tuple{Any}) = Iterators.TupleOrBottom(_broadcast_getindex_eltype(t[1]))
+eltypes(t::Tuple{Any,Any}) = Iterators.TupleOrBottom(_broadcast_getindex_eltype(t[1]), _broadcast_getindex_eltype(t[2]))
+# eltypes(t::Tuple) = (TT = eltypes(tail(t)); TT === Union{} ? Union{} : Iterators.TupleOrBottom(_broadcast_getindex_eltype(t[1]), TT.parameters...))
+eltypes(t::Tuple) = Iterators.TupleOrBottom(ntuple(i -> _broadcast_getindex_eltype(t[i]), Val(length(t)))...)
 
 # Inferred eltype of result of broadcast(f, args...)
-combine_eltypes(f, args::Tuple) =
-    promote_typejoin_union(Base._return_type(f, eltypes(args)))
+function combine_eltypes(f, args::Tuple)
+    argT = eltypes(args)
+    argT === Union{} && return Union{}
+    return promote_typejoin_union(Base._return_type(f, argT))
+end
 
 ## Broadcasting core
 

base/compiler/abstractinterpretation.jl

@@ -541,6 +541,8 @@ function abstract_call_method(interp::AbstractInterpreter,
         add_remark!(interp, sv, "Refusing to infer into `depwarn`")
         return MethodCallResult(Any, false, false, nothing, Effects())
     end
+    sigtuple = unwrap_unionall(sig)
+    sigtuple isa DataType || return MethodCallResult(Any, false, false, nothing, Effects())
 
     # Limit argument type tuple growth of functions:
     # look through the parents list to see if there's a call to the same method
@@ -577,7 +579,6 @@ function abstract_call_method(interp::AbstractInterpreter,
     washardlimit = hardlimit
 
     if topmost !== nothing
-        sigtuple = unwrap_unionall(sig)::DataType
         msig = unwrap_unionall(method.sig)::DataType
         spec_len = length(msig.parameters) + 1
         ls = length(sigtuple.parameters)
@@ -1394,7 +1395,11 @@ function precise_container_type(interp::AbstractInterpreter, @nospecialize(itft)
             va = isvarargtype(last)
             elts = Any[ fieldtype(tti0, i) for i = 1:len ]
             if va
-                elts[len] = Vararg{elts[len]}
+                if elts[len] === Union{}
+                    pop!(elts)
+                else
+                    elts[len] = Vararg{elts[len]}
+                end
             end
             return AbstractIterationResult(elts, nothing)
         end
@@ -1403,6 +1408,9 @@ function precise_container_type(interp::AbstractInterpreter, @nospecialize(itft)
     elseif tti0 === Any
         return AbstractIterationResult(Any[Vararg{Any}], nothing, Effects())
     elseif tti0 <: Array
+        if eltype(tti0) === Union{}
+            return AbstractIterationResult(Any[], nothing)
+        end
         return AbstractIterationResult(Any[Vararg{eltype(tti0)}], nothing)
     else
         return abstract_iteration(interp, itft, typ, sv)
@@ -2115,7 +2123,7 @@ end
 
 function sp_type_rewrap(@nospecialize(T), linfo::MethodInstance, isreturn::Bool)
     isref = false
-    if T === Bottom
+    if unwrapva(T) === Bottom
         return Bottom
     elseif isa(T, Type)
         if isa(T, DataType) && (T::DataType).name === _REF_NAME
@@ -2152,8 +2160,13 @@ end
 function abstract_eval_cfunction(interp::AbstractInterpreter, e::Expr, vtypes::Union{VarTable,Nothing}, sv::AbsIntState)
     f = abstract_eval_value(interp, e.args[2], vtypes, sv)
     # rt = sp_type_rewrap(e.args[3], sv.linfo, true)
-    at = Any[ sp_type_rewrap(argt, frame_instance(sv), false) for argt in e.args[4]::SimpleVector ]
-    pushfirst!(at, f)
+    atv = e.args[4]::SimpleVector
+    at = Vector{Any}(undef, length(atv) + 1)
+    at[1] = f
+    for i = 1:length(atv)
+        at[i + 1] = sp_type_rewrap(at[i], frame_instance(sv), false)
+        at[i + 1] === Bottom && return
+    end
     # this may be the wrong world for the call,
     # but some of the result is likely to be valid anyways
     # and that may help generate better codegen
@@ -2370,7 +2383,7 @@ function abstract_eval_statement_expr(interp::AbstractInterpreter, e::Expr, vtyp
                 end))
                 nothrow = isexact
                 t = Const(ccall(:jl_new_structt, Any, (Any, Any), t, at.val))
-            elseif (isa(at, PartialStruct) && at ⊑ᵢ Tuple && n == length(at.fields::Vector{Any}) &&
+            elseif (isa(at, PartialStruct) && at ⊑ᵢ Tuple && n > 0 && n == length(at.fields::Vector{Any}) && !isvarargtype(at.fields[end]) &&
                     (let t = t, at = at, ⊑ᵢ = ⊑ᵢ
                         all(i::Int->(at.fields::Vector{Any})[i] ⊑ᵢ fieldtype(t, i), 1:n)
                     end))

base/compiler/inferenceresult.jl

@@ -117,9 +117,9 @@ function most_general_argtypes(method::Union{Method, Nothing}, @nospecialize(spe
     # to the appropriate `Tuple` type or `PartialStruct` instance.
     if !toplevel && isva
         if specTypes::Type == Tuple
+            linfo_argtypes = Any[Any for i = 1:nargs]
             if nargs > 1
-                linfo_argtypes = Any[Any for i = 1:nargs]
-                linfo_argtypes[end] = Vararg{Any}
+                linfo_argtypes[end] = Tuple
             end
             vargtype = Tuple
         else

base/compiler/ssair/inlining.jl

@@ -1170,7 +1170,7 @@ function handle_invoke_call!(todo::Vector{Pair{Int,Any}},
         if isa(result, ConstPropResult)
             mi = result.result.linfo
             validate_sparams(mi.sparam_vals) || return nothing
-            if argtypes_to_type(argtypes) <: mi.def.sig
+            if Union{} !== argtypes_to_type(argtypes) <: mi.def.sig
                 item = resolve_todo(mi, result.result, argtypes, info, flag, state; invokesig)
                 handle_single_case!(todo, ir, idx, stmt, item, OptimizationParams(state.interp), true)
                 return nothing

base/compiler/tfuncs.jl

@@ -1904,7 +1904,15 @@ add_tfunc(apply_type, 1, INT_INF, apply_type_tfunc, 10)
 # convert the dispatch tuple type argtype to the real (concrete) type of
 # the tuple of those values
 function tuple_tfunc(𝕃::AbstractLattice, argtypes::Vector{Any})
+    isempty(argtypes) && return Const(())
     argtypes = anymap(widenslotwrapper, argtypes)
+    if isvarargtype(argtypes[end]) && unwrapva(argtypes[end]) === Union{}
+        # Drop the Vararg in Tuple{...,Vararg{Union{}}} since it must be length 0.
+        # If there is a Vararg num also, it must be a TypeVar, and it must be
+        # zero, but that generally shouldn't show up here, since it implies a
+        # UnionAll context is missing around this.
+        pop!(argtypes)
+    end
     all_are_const = true
     for i in 1:length(argtypes)
         if !isa(argtypes[i], Const)
@@ -1947,6 +1955,8 @@ function tuple_tfunc(𝕃::AbstractLattice, argtypes::Vector{Any})
                 params[i] = x
             elseif !isvarargtype(x) && hasintersect(x, Type)
                 params[i] = Union{x, Type}
+            elseif x === Union{}
+                return Bottom # argtypes is malformed, but try not to crash
             else
                 params[i] = x
             end

base/compiler/typeutils.jl

@@ -187,6 +187,7 @@ function typesubtract(@nospecialize(a), @nospecialize(b), max_union_splitting::I
                             bp = b.parameters[i]
                             (isvarargtype(ap) || isvarargtype(bp)) && return a
                             ta[i] = typesubtract(ap, bp, min(2, max_union_splitting))
+                            ta[i] === Union{} && return Union{}
                             return Tuple{ta...}
                         end
                     end

base/iterators.jl

@@ -12,7 +12,7 @@ using .Base:
     @inline, Pair, Pairs, AbstractDict, IndexLinear, IndexStyle, AbstractVector, Vector,
     SizeUnknown, HasLength, HasShape, IsInfinite, EltypeUnknown, HasEltype, OneTo,
     @propagate_inbounds, @isdefined, @boundscheck, @inbounds, Generator,
-    AbstractRange, AbstractUnitRange, UnitRange, LinearIndices,
+    AbstractRange, AbstractUnitRange, UnitRange, LinearIndices, TupleOrBottom,
     (:), |, +, -, *, !==, !, ==, !=, <=, <, >, >=, missing,
     any, _counttuple, eachindex, ntuple, zero, prod, reduce, in, firstindex, lastindex,
     tail, fieldtypes, min, max, minimum, zero, oneunit, promote, promote_shape
@@ -209,7 +209,7 @@ size(e::Enumerate) = size(e.itr)
 end
 last(e::Enumerate) = (length(e.itr), e.itr[end])
 
-eltype(::Type{Enumerate{I}}) where {I} = Tuple{Int, eltype(I)}
+eltype(::Type{Enumerate{I}}) where {I} = TupleOrBottom(Int, eltype(I))
 
 IteratorSize(::Type{Enumerate{I}}) where {I} = IteratorSize(I)
 IteratorEltype(::Type{Enumerate{I}}) where {I} = IteratorEltype(I)
@@ -394,7 +394,7 @@ _promote_tuple_shape((m,)::Tuple{Integer}, (n,)::Tuple{Integer}) = (min(m, n),)
 _promote_tuple_shape(a, b) = promote_shape(a, b)
 _promote_tuple_shape(a, b...) = _promote_tuple_shape(a, _promote_tuple_shape(b...))
 _promote_tuple_shape(a) = a
-eltype(::Type{Zip{Is}}) where {Is<:Tuple} = Tuple{map(eltype, fieldtypes(Is))...}
+eltype(::Type{Zip{Is}}) where {Is<:Tuple} = TupleOrBottom(map(eltype, fieldtypes(Is))...)
 #eltype(::Type{Zip{Tuple{}}}) = Tuple{}
 #eltype(::Type{Zip{Tuple{A}}}) where {A} = Tuple{eltype(A)}
 #eltype(::Type{Zip{Tuple{A, B}}}) where {A, B} = Tuple{eltype(A), eltype(B)}
@@ -1072,8 +1072,7 @@ end
 
 eltype(::Type{ProductIterator{I}}) where {I} = _prod_eltype(I)
 _prod_eltype(::Type{Tuple{}}) = Tuple{}
-_prod_eltype(::Type{I}) where {I<:Tuple} =
-    Tuple{ntuple(n -> eltype(fieldtype(I, n)), _counttuple(I)::Int)...}
+_prod_eltype(::Type{I}) where {I<:Tuple} = TupleOrBottom(ntuple(n -> eltype(fieldtype(I, n)), _counttuple(I)::Int)...)
 
 iterate(::ProductIterator{Tuple{}}) = (), true
 iterate(::ProductIterator{Tuple{}}, state) = nothing
@@ -1442,6 +1441,7 @@ end
 function _approx_iter_type(itrT::Type, vstate::Type)
     vstate <: Union{Nothing, Tuple{Any, Any}} || return Any
     vstate <: Union{} && return Union{}
+    itrT <: Union{} && return Union{}
     nextvstate = Base._return_type(doiterate, Tuple{itrT, vstate})
     return (nextvstate <: vstate ? vstate : Any)
 end

base/promotion.jl

@@ -472,6 +472,11 @@ else
     _return_type(@nospecialize(f), @nospecialize(t)) = Any
 end
 
+function TupleOrBottom(tt...)
+    any(p -> p === Union{}, tt) && return Union{}
+    return Tuple{tt...}
+end
+
 """
     promote_op(f, argtypes...)
 
@@ -483,7 +488,12 @@ Guess what an appropriate container eltype would be for storing results of
     the container eltype on the type of the actual elements. Only in the absence of any
     elements (for an empty result container), it may be unavoidable to call `promote_op`.
 """
-promote_op(f, S::Type...) = _return_type(f, Tuple{S...})
+function promote_op(f, S::Type...)
+    argT = TupleOrBottom(S...)
+    argT === Union{} && return Union{}
+    return _return_type(f, argT)
+end
+
 
 ## catch-alls to prevent infinite recursion when definitions are missing ##
 

base/slicearray.jl

@@ -40,7 +40,8 @@ unitaxis(::AbstractArray) = Base.OneTo(1)
 
 function Slices(A::P, slicemap::SM, ax::AX) where {P,SM,AX}
     N = length(ax)
-    S = Base._return_type(view, Tuple{P, map((a,l) -> l === (:) ? Colon : eltype(a), axes(A), slicemap)...})
+    argT = map((a,l) -> l === (:) ? Colon : eltype(a), axes(A), slicemap)
+    S = Base.promote_op(view, P, argT...)
     Slices{P,SM,AX,S,N}(A, slicemap, ax)
 end
 

src/builtins.c

@@ -1320,7 +1320,7 @@ JL_CALLABLE(jl_f_apply_type)
                 jl_type_error_rt("Tuple", "parameter", (jl_value_t*)jl_type_type, pi);
             }
         }
-        return (jl_value_t*)jl_apply_tuple_type_v(&args[1], nargs-1);
+        return jl_apply_tuple_type_v(&args[1], nargs-1);
     }
     else if (args[0] == (jl_value_t*)jl_uniontype_type) {
         // Union{} has extra restrictions, so it needs to be checked after