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Allow effect inference of eltype(::Tuple) #48322

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Merged
merged 1 commit into from
Jan 18, 2023
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Allow effect inference of eltype(::Tuple) #48322

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15 changes: 15 additions & 0 deletions base/tuple.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -234,6 +234,21 @@ function _compute_eltype(@nospecialize t)
end
end

# We'd like to be able to infer eltype(::Tuple), which needs to be able to
# look at these four methods:
#
# julia> methods(Base.eltype, Tuple{Type{<:Tuple}})
# 4 methods for generic function "eltype" from Base:
# [1] eltype(::Type{Union{}})
# @ abstractarray.jl:234
# [2] eltype(::Type{Tuple{}})
# @ tuple.jl:199
# [3] eltype(t::Type{<:Tuple{Vararg{E}}}) where E
# @ tuple.jl:200
# [4] eltype(t::Type{<:Tuple})
# @ tuple.jl:209
typeof(function eltype end).name.max_methods = UInt8(4)

# version of tail that doesn't throw on empty tuples (used in array indexing)
safe_tail(t::Tuple) = tail(t)
safe_tail(t::Tuple{}) = ()
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1 change: 1 addition & 0 deletions test/tuple.jl
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Expand Up @@ -782,3 +782,4 @@ namedtup = (;a=1, b=2, c=3)

# Make sure that tuple iteration is foldable
@test Core.Compiler.is_foldable(Base.infer_effects(iterate, Tuple{NTuple{4, Float64}, Int}))
@test Core.Compiler.is_foldable(Base.infer_effects(eltype, Tuple{Tuple}))