Reland "add more methods and tests for reductions over empty arrays"

Refs #29919
Refs #52003

This reverts commit 2a84214.

base/reduce.jl

@@ -338,7 +338,8 @@ reduce_empty(::typeof(*), ::Type{T}) where {T} = one(T)
 reduce_empty(::typeof(*), ::Type{<:AbstractChar}) = ""
 reduce_empty(::typeof(&), ::Type{Bool}) = true
 reduce_empty(::typeof(|), ::Type{Bool}) = false
-
+reduce_empty(::typeof(max), T) = typemin(T)
+reduce_empty(::typeof(min), T) = typemax(T)
 reduce_empty(::typeof(add_sum), ::Type{T}) where {T} = reduce_empty(+, T)
 reduce_empty(::typeof(add_sum), ::Type{T}) where {T<:BitSignedSmall}  = zero(Int)
 reduce_empty(::typeof(add_sum), ::Type{T}) where {T<:BitUnsignedSmall} = zero(UInt)
@@ -362,8 +363,10 @@ mapreduce_empty(::typeof(identity), op, T) = reduce_empty(op, T)
 mapreduce_empty(::typeof(abs), op, T)      = abs(reduce_empty(op, T))
 mapreduce_empty(::typeof(abs2), op, T)     = abs2(reduce_empty(op, T))
 
-mapreduce_empty(f::typeof(abs),  ::typeof(max), T) = abs(zero(T))
-mapreduce_empty(f::typeof(abs2), ::typeof(max), T) = abs2(zero(T))
+mapreduce_empty(::typeof(abs),  ::typeof(max), T) = abs(zero(T))
+mapreduce_empty(::typeof(abs),  ::typeof(min), T) = typemax(abs(zero(T)))
+mapreduce_empty(::typeof(abs2), ::typeof(max), T) = abs2(zero(T))
+mapreduce_empty(::typeof(abs2),  ::typeof(min), T) = typemax(abs2(zero(T)))
 
 # For backward compatibility:
 mapreduce_empty_iter(f, op, itr, ItrEltype) =

test/reduce.jl

@@ -91,6 +91,10 @@ end
 @test mapreduce(abs2, *, Float64[]) === 1.0
 @test mapreduce(abs2, max, Float64[]) === 0.0
 @test mapreduce(abs, max, Float64[]) === 0.0
+@test mapreduce(abs2, min, Float64[]) === Inf
+@test mapreduce(abs, min, Float64[]) === Inf
+@test_throws ArgumentError mapreduce(abs2, &, Float64[])
+@test_throws ArgumentError mapreduce(abs2, |, Float64[])
 @test_throws "reducing over an empty collection is not allowed" mapreduce(abs2, &, Float64[])
 @test_throws str -> !occursin("Closest candidates are", str) mapreduce(abs2, &, Float64[])
 @test_throws "reducing over an empty collection is not allowed" mapreduce(abs2, |, Float64[])
@@ -246,8 +250,8 @@ prod2(itr) = invoke(prod, Tuple{Any}, itr)
 
 # maximum & minimum & extrema
 
-@test_throws "reducing over an empty" maximum(Int[])
-@test_throws "reducing over an empty" minimum(Int[])
+@test maximum(Int[]) === typemin(Int)
+@test minimum(Int[]) === typemax(Int)
 @test_throws "reducing over an empty" extrema(Int[])
 
 @test maximum(Int[]; init=-1) == -1
@@ -258,6 +262,21 @@ prod2(itr) = invoke(prod, Tuple{Any}, itr)
 @test minimum(sin, []; init=1) == 1
 @test extrema(sin, []; init=(1, -1)) == (1, -1)
 
+@test maximum(Float64[]) === -Inf
+@test minimum(Float64[]) === +Inf
+
+@test maximum(Float32[]) === -Inf32
+@test minimum(Float32[]) === +Inf32
+
+@test maximum(abs, Int[]) === 0
+@test minimum(abs, Int[]) === typemax(Int)
+
+@test maximum(abs, Float64[]) === 0.0
+@test minimum(abs, Float64[]) === +Inf
+
+@test maximum(abs, Float32[]) === 0.0f0
+@test minimum(abs, Float32[]) === +Inf32
+
 @test maximum(5) == 5
 @test minimum(5) == 5
 @test extrema(5) == (5, 5)