remove test for empty tuple for mean

[KristofferC]

JuliaLang/julia#35803 broke the tests for Statistics because an ambiguity was fixed:

julia> using Statistics

julia> mean(())
ERROR: MethodError: reduce_empty(::typeof(Base.add_sum), ::Type{Union{}}) is ambiguous. Candidates:
  reduce_empty(::typeof(Base.add_sum), ::Type{T}) where T<:Union{Int16, Int32, Int8} in Base at reduce.jl:314
  reduce_empty(::typeof(Base.add_sum), ::Type{T}) where T<:Union{UInt16, UInt32, UInt8} in Base at reduce.jl:315
Possible fix, define
  reduce_empty(::typeof(Base.add_sum), ::Type{Union{}})

which after that PR is now (more correctly):

julia> mean(())
ERROR: ArgumentError: reducing over an empty collection is not allowed

This test doesn't seem to do anything so just delete it.

[timholy]

Without that PR it's

julia> mean(())
ERROR: MethodError: zero(::Type{Union{}}) is ambiguous. Candidates:
  zero(::Type{var"#s822"} where var"#s822"<:AbstractIrrational) in Base at irrationals.jl:149
  zero(::Union{Type{P}, P}) where P<:Dates.Period in Dates at /home/tim/src/julia-master/usr/share/julia/stdlib/v1.6/Dates/src/periods.jl:53
  zero(::Type{T}) where T<:Number in Base at number.jl:242
Possible fix, define
  zero(::Type{Union{}})
Stacktrace:
 [1] reduce_empty(::typeof(+), ::Type{T} where T) at ./reduce.jl:310
 [2] mapreduce_empty(::typeof(identity), ::Function, ::Type{T} where T) at ./reduce.jl:339
 [3] reduce_empty(::Base.MappingRF{typeof(identity),typeof(+)}, ::Type{T} where T) at ./reduce.jl:325
 [4] reduce_empty_iter at ./reduce.jl:351 [inlined]
 [5] mapreduce_empty_iter(::Function, ::Function, ::Tuple{}, ::Base.HasEltype) at ./reduce.jl:347
 [6] mean(::typeof(identity), ::Tuple{}) at /home/tim/src/julia-master/usr/share/julia/stdlib/v1.6/Statistics/src/Statistics.jl:64
 [7] mean(::Tuple{}) at /home/tim/src/julia-master/usr/share/julia/stdlib/v1.6/Statistics/src/Statistics.jl:44
 [8] top-level scope at REPL[3]:1

This might be a consequence of

julia> Base.IteratorEltype(())
Base.HasEltype()

I think for Julia 1.6 we should explore

diff --git a/base/generator.jl b/base/generator.jl
index 7cd075d255..cfe520070f 100644
--- a/base/generator.jl
+++ b/base/generator.jl
@@ -126,3 +126,6 @@ IteratorEltype(::Type) = HasEltype()  # HasEltype is the default
 IteratorEltype(::Type{Generator{I,T}}) where {I,T} = EltypeUnknown()
 
 IteratorEltype(::Type{Any}) = EltypeUnknown()
+
+IteratorEltype(::Type{<:Tuple}) = EltypeUnknown()
+IteratorEltype(::Type{Tuple{T,Vararg{T}}}) where {T} = HasEltype()

Running local tests now to see what breaks.

[tkf]

👍 for this change

I think reduce_empty should handle Union{} correctly. So, a part of JuliaLang/julia#35733 does make sense even only from the point of view of correctness. For example, sum(Union{}[]) should not throw MethodError.

[tkf]

This is required for JuliaLang/julia#35843

[nalimilan]

Sorry for not responding before, but can you add a test for the new behavior conditional on the version? That sounds like a legitimate case to test.

[tkf]

@nalimilan see https://github.com/JuliaLang/Statistics.jl/pull/40