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make eltype throw for some known noniterators #58122

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make eltype throw for some known noniterators #58122

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7e264aa
make `eltype` throw for some known noniterators
nsajko Apr 15, 2025
ca285fe
make `eltype(missing)` work again
nsajko May 8, 2025
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3 changes: 3 additions & 0 deletions base/missing.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -457,3 +457,6 @@ macro coalesce(args...)
end
return :(let val; $expr; end)
end

eltype(::Missing) = Any # imprecise but correct approximation for the element type of an unknown iterator
eltype(::Type{<:Union{Nothing, Missing}}) = throw(ArgumentError("not an iterator, doesn't have an element type"))
8 changes: 8 additions & 0 deletions test/iterators.jl
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Expand Up @@ -1133,6 +1133,14 @@ end
end
end

@testset "noniterator has no element type" begin
for x ∈ (nothing, missing)
@test_throws ArgumentError eltype(x)
@test_throws ArgumentError eltype(typeof(x))
end
@test_throws ArgumentError eltype(Union{Nothing, Missing})
end

@testset "Iterators docstrings" begin
@test isempty(Docs.undocumented_names(Iterators))
end
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