Merge pull request #49470 from JuliaLang/jn/morespecific-bottom

reland: add morespecific rule for Type{Union{}}

Author: vtjnash

NEWS.md

@@ -8,6 +8,12 @@ Language changes
 ----------------
 
 * When a task forks a child, the parent task's task-local RNG (random number generator) is no longer affected. The seeding of child based on the parent task also takes a more disciplined approach to collision resistance, using a design based on the SplitMix and DotMix splittable RNG schemes ([#49110]).
+* A new morespecific rule for methods resolves ambiguities containing Union{} in favor of
+  the method defined explicitly to handle the Union{} argument. This makes it possible to
+  define methods to explicitly handle Union{} without the ambiguities that commonly would
+  result previously. This also lets the runtime optimize certain method lookups in a way
+  that significantly improves load and inference times for heavily overloaded methods that
+  dispatch on Types (such as traits and constructors).
 
 Compiler/Runtime improvements
 -----------------------------

base/abstractarray.jl

@@ -183,11 +183,13 @@ CartesianIndex{2}
      For arrays, this function requires at least Julia 1.2.
 """
 keytype(a::AbstractArray) = keytype(typeof(a))
+keytype(::Type{Union{}}, slurp...) = eltype(Union{})
 
 keytype(A::Type{<:AbstractArray}) = CartesianIndex{ndims(A)}
 keytype(A::Type{<:AbstractVector}) = Int
 
 valtype(a::AbstractArray) = valtype(typeof(a))
+valtype(::Type{Union{}}, slurp...) = eltype(Union{})
 
 """
     valtype(T::Type{<:AbstractArray})
@@ -232,7 +234,7 @@ UInt8
 ```
 """
 eltype(::Type) = Any
-eltype(::Type{Bottom}) = throw(ArgumentError("Union{} does not have elements"))
+eltype(::Type{Bottom}, slurp...) = throw(ArgumentError("Union{} does not have elements"))
 eltype(x) = eltype(typeof(x))
 eltype(::Type{<:AbstractArray{E}}) where {E} = @isdefined(E) ? E : Any
 
@@ -268,6 +270,7 @@ julia> ndims(A)
 """
 ndims(::AbstractArray{T,N}) where {T,N} = N
 ndims(::Type{<:AbstractArray{<:Any,N}}) where {N} = N
+ndims(::Type{Union{}}, slurp...) = throw(ArgumentError("Union{} does not have elements"))
 
 """
     length(collection) -> Integer

base/array.jl

@@ -252,7 +252,10 @@ function bitsunionsize(u::Union)
     return sz
 end
 
+# Deprecate this, as it seems to have no documented meaning and is unused here,
+# but is frequently accessed in packages
 elsize(@nospecialize _::Type{A}) where {T,A<:Array{T}} = aligned_sizeof(T)
+elsize(::Type{Union{}}, slurp...) = 0
 sizeof(a::Array) = Core.sizeof(a)
 
 function isassigned(a::Array, i::Int...)

base/arrayshow.jl

@@ -540,10 +540,12 @@ end
 # returning Any, as this would cause incorrect printing in e.g. `Vector[Any[1]]`,
 # because eltype(Vector) == Any so `Any` wouldn't be printed in `Any[1]`)
 typeinfo_eltype(typeinfo) = nothing # element type not precisely known
+typeinfo_eltype(typeinfo::Type{Union{}}, slurp...) = nothing
 typeinfo_eltype(typeinfo::Type{<:AbstractArray{T}}) where {T} = eltype(typeinfo)
 typeinfo_eltype(typeinfo::Type{<:AbstractDict{K,V}}) where {K,V} = eltype(typeinfo)
 typeinfo_eltype(typeinfo::Type{<:AbstractSet{T}}) where {T} = eltype(typeinfo)
 
+
 # types that can be parsed back accurately from their un-decorated representations
 function typeinfo_implicit(@nospecialize(T))
     if T === Float64 || T === Int || T === Char || T === String || T === Symbol ||

base/boot.jl

@@ -258,9 +258,17 @@ UnionAll(v::TypeVar, @nospecialize(t)) = ccall(:jl_type_unionall, Any, (Any, Any
 
 const Vararg = ccall(:jl_toplevel_eval_in, Any, (Any, Any), Core, _expr(:new, TypeofVararg))
 
-# let the compiler assume that calling Union{} as a constructor does not need
-# to be considered ever (which comes up often as Type{<:T})
-Union{}(a...) = throw(MethodError(Union{}, a))
+# dispatch token indicating a kwarg (keyword sorter) call
+function kwcall end
+# deprecated internal functions:
+kwfunc(@nospecialize(f)) = kwcall
+kwftype(@nospecialize(t)) = typeof(kwcall)
+
+# Let the compiler assume that calling Union{} as a constructor does not need
+# to be considered ever (which comes up often as Type{<:T} inference, and
+# occasionally in user code from eltype).
+Union{}(a...) = throw(ArgumentError("cannot construct a value of type Union{} for return result"))
+kwcall(kwargs, ::Type{Union{}}, a...) = Union{}(a...)
 
 Expr(@nospecialize args...) = _expr(args...)
 
@@ -369,12 +377,6 @@ include(m::Module, fname::String) = ccall(:jl_load_, Any, (Any, Any), m, fname)
 
 eval(m::Module, @nospecialize(e)) = ccall(:jl_toplevel_eval_in, Any, (Any, Any), m, e)
 
-# dispatch token indicating a kwarg (keyword sorter) call
-function kwcall end
-# deprecated internal functions:
-kwfunc(@nospecialize(f)) = kwcall
-kwftype(@nospecialize(t)) = typeof(kwcall)
-
 mutable struct Box
     contents::Any
     Box(@nospecialize(x)) = new(x)

base/broadcast.jl

@@ -34,6 +34,9 @@ that you may be able to leverage; see the
 """
 abstract type BroadcastStyle end
 
+struct Unknown <: BroadcastStyle end
+BroadcastStyle(::Type{Union{}}, slurp...) = Unknown()  # ambiguity resolution
+
 """
 `Broadcast.Style{C}()` defines a [`BroadcastStyle`](@ref) signaling through the type
 parameter `C`. You can use this as an alternative to creating custom subtypes of `BroadcastStyle`,
@@ -45,9 +48,6 @@ struct Style{T} <: BroadcastStyle end
 
 BroadcastStyle(::Type{<:Tuple}) = Style{Tuple}()
 
-struct Unknown <: BroadcastStyle end
-BroadcastStyle(::Type{Union{}}) = Unknown()  # ambiguity resolution
-
 """
 `Broadcast.AbstractArrayStyle{N} <: BroadcastStyle` is the abstract supertype for any style
 associated with an `AbstractArray` type.

base/complex.jl

@@ -120,6 +120,7 @@ Float64
 real(T::Type) = typeof(real(zero(T)))
 real(::Type{T}) where {T<:Real} = T
 real(C::Type{<:Complex}) = fieldtype(C, 1)
+real(::Type{Union{}}, slurp...) = Union{}(im)
 
 """
     isreal(x) -> Bool

base/essentials.jl

@@ -310,13 +310,8 @@ See also: [`round`](@ref), [`trunc`](@ref), [`oftype`](@ref), [`reinterpret`](@r
 """
 function convert end
 
-# make convert(::Type{<:Union{}}, x::T) intentionally ambiguous for all T
-# so it will never get called or invalidated by loading packages
-# with carefully chosen types that won't have any other convert methods defined
-convert(T::Type{<:Core.IntrinsicFunction}, x) = throw(MethodError(convert, (T, x)))
-convert(T::Type{<:Nothing}, x) = throw(MethodError(convert, (Nothing, x)))
-convert(::Type{T}, x::T) where {T<:Core.IntrinsicFunction} = x
-convert(::Type{T}, x::T) where {T<:Nothing} = x
+# ensure this is never ambiguous, and therefore fast for lookup
+convert(T::Type{Union{}}, x...) = throw(ArgumentError("cannot convert a value to Union{} for assignment"))
 
 convert(::Type{Type}, x::Type) = x # the ssair optimizer is strongly dependent on this method existing to avoid over-specialization
                                    # in the absence of inlining-enabled
@@ -540,6 +535,7 @@ Neither `convert` nor `cconvert` should take a Julia object and turn it into a `
 function cconvert end
 
 cconvert(T::Type, x) = x isa T ? x : convert(T, x) # do the conversion eagerly in most cases
+cconvert(::Type{Union{}}, x...) = convert(Union{}, x...)
 cconvert(::Type{<:Ptr}, x) = x # but defer the conversion to Ptr to unsafe_convert
 unsafe_convert(::Type{T}, x::T) where {T} = x # unsafe_convert (like convert) defaults to assuming the convert occurred
 unsafe_convert(::Type{T}, x::T) where {T<:Ptr} = x  # to resolve ambiguity with the next method

base/float.jl

@@ -310,6 +310,7 @@ Float64
 """
 float(::Type{T}) where {T<:Number} = typeof(float(zero(T)))
 float(::Type{T}) where {T<:AbstractFloat} = T
+float(::Type{Union{}}, slurp...) = Union{}(0.0)
 
 """
     unsafe_trunc(T, x)

base/generator.jl

@@ -92,13 +92,13 @@ Base.HasLength()
 """
 IteratorSize(x) = IteratorSize(typeof(x))
 IteratorSize(::Type) = HasLength()  # HasLength is the default
+IteratorSize(::Type{Union{}}, slurp...) = throw(ArgumentError("Union{} does not have elements"))
+IteratorSize(::Type{Any}) = SizeUnknown()
 
 IteratorSize(::Type{<:Tuple}) = HasLength()
 IteratorSize(::Type{<:AbstractArray{<:Any,N}})  where {N} = HasShape{N}()
 IteratorSize(::Type{Generator{I,F}}) where {I,F} = IteratorSize(I)
 
-IteratorSize(::Type{Any}) = SizeUnknown()
-
 haslength(iter) = IteratorSize(iter) isa Union{HasShape, HasLength}
 
 abstract type IteratorEltype end
@@ -126,7 +126,7 @@ Base.HasEltype()
 """
 IteratorEltype(x) = IteratorEltype(typeof(x))
 IteratorEltype(::Type) = HasEltype()  # HasEltype is the default
+IteratorEltype(::Type{Union{}}, slurp...) = throw(ArgumentError("Union{} does not have elements"))
+IteratorEltype(::Type{Any}) = EltypeUnknown()
 
 IteratorEltype(::Type{Generator{I,T}}) where {I,T} = EltypeUnknown()
-
-IteratorEltype(::Type{Any}) = EltypeUnknown()