remove fallback eltype that returns Any for all types

Author: JeffBezanson

NEWS.md

@@ -1147,6 +1147,9 @@ Deprecated or removed
 
   * `signif` has been deprecated in favor of the `sigdigits` keyword argument to `round`.
 
+  * The fallback definition of `eltype` that returned `Any` for any type is deprecated.
+    Code should be restructured to avoid calling `eltype` on types that might not support it ([#26852]).
+
 Command-line option changes
 ---------------------------
 

base/array.jl

@@ -84,7 +84,12 @@ julia> eltype(fill(0x1, (2,2)))
 UInt8
 ```
 """
-eltype(::Type) = Any
+function eltype(t::Type)
+    # TODO: change post-0.7
+    #throw(MethodError(eltype, (t,)))
+    depwarn("The fallback definition of `eltype` is deprecated; either define it for your type or avoid calling it on types that might not support it.", :eltype)
+    return Any
+end
 eltype(::Type{Bottom}) = throw(ArgumentError("Union{} does not have elements"))
 eltype(x) = eltype(typeof(x))
 

base/compiler/abstractinterpretation.jl

@@ -350,7 +350,7 @@ function precise_container_type(@nospecialize(arg), @nospecialize(typ), vtypes::
         else
             return Any[ p for p in tti0.parameters ]
         end
-    elseif tti0 <: Array
+    elseif tti0 isa Type{<:Array{T}} where T
         return Any[Vararg{eltype(tti0)}]
     else
         return Any[abstract_iteration(typ, vtypes, sv)]

base/dict.jl

@@ -134,7 +134,7 @@ Dict(ps::Pair...)                  = Dict(ps)
 
 function Dict(kv)
     try
-        dict_with_eltype((K, V) -> Dict{K, V}, kv, eltype(kv))
+        dict_with_eltype((K, V) -> Dict{K, V}, kv, IteratorEltype(typeof(kv)) isa HasEltype ? eltype(kv) : Any)
     catch e
         if !applicable(start, kv) || !all(x->isa(x,Union{Tuple,Pair}),kv)
             throw(ArgumentError("Dict(kv): kv needs to be an iterator of tuples or pairs"))

base/weakkeydict.jl

@@ -58,7 +58,7 @@ WeakKeyDict(ps::Pair...)                            = WeakKeyDict{Any,Any}(ps)
 
 function WeakKeyDict(kv)
     try
-        Base.dict_with_eltype((K, V) -> WeakKeyDict{K, V}, kv, eltype(kv))
+        Base.dict_with_eltype((K, V) -> WeakKeyDict{K, V}, kv, IteratorEltype(typeof(kv)) isa HasEltype ? eltype(kv) : Any)
     catch e
         if !applicable(start, kv) || !all(x->isa(x,Union{Tuple,Pair}),kv)
             throw(ArgumentError("WeakKeyDict(kv): kv needs to be an iterator of tuples or pairs"))

stdlib/Distributed/src/cluster.jl

@@ -42,8 +42,7 @@ mutable struct WorkerConfig
 
     function WorkerConfig()
         wc = new()
-        for n in 1:length(WorkerConfig.types)
-            T = eltype(fieldtype(WorkerConfig, n))
+        for n in 1:fieldcount(WorkerConfig)
             setfield!(wc, n, nothing)
         end
         wc

stdlib/Random/src/Random.jl

@@ -132,7 +132,7 @@ struct SamplerTrivial{T,E} <: Sampler{E}
     self::T
 end
 
-SamplerTrivial(x::T) where {T} = SamplerTrivial{T,eltype(T)}(x)
+SamplerTrivial(x::T) where {T} = SamplerTrivial{T,Any}(x)
 
 Sampler(::AbstractRNG, x, ::Repetition) = SamplerTrivial(x)
 
@@ -144,14 +144,14 @@ struct SamplerSimple{T,S,E} <: Sampler{E}
     data::S
 end
 
-SamplerSimple(x::T, data::S) where {T,S} = SamplerSimple{T,S,eltype(T)}(x, data)
+SamplerSimple(x::T, data::S) where {T,S} = SamplerSimple{T,S,Any}(x, data)
 
 Base.getindex(sp::SamplerSimple) = sp.self
 
 # simple sampler carrying a (type) tag T and data
 struct SamplerTag{T,S,E} <: Sampler{E}
     data::S
-    SamplerTag{T}(s::S) where {T,S} = new{T,S,eltype(T)}(s)
+    SamplerTag{T}(s::S) where {T,S} = new{T,S,Any}(s)
 end
 
 

test/missing.jl

@@ -358,7 +358,8 @@ end
     @test collect(x) isa Vector{Int}
 
     x = skipmissing(v for v in [missing, 1, missing, 2, 4])
-    @test eltype(x) === Any
+    # TODO: enable in v0.7
+    #@test_throws MethodError eltype(x)
     @test collect(x) == [1, 2, 4]
     @test collect(x) isa Vector{Int}
 end