combine reduce_empty methods for Union{} eltypes

With #49470, these can all be dispatched to the same method now,
avoiding unnecessary code duplication for this case.

base/errorshow.jl

@@ -258,9 +258,6 @@ function showerror(io::IO, ex::MethodError)
     if f === Base.convert && length(arg_types_param) == 2 && !is_arg_types
         f_is_function = true
         show_convert_error(io, ex, arg_types_param)
-    elseif f === mapreduce_empty || f === reduce_empty
-        print(io, "reducing over an empty collection is not allowed; consider supplying `init` to the reducer")
-        show_candidates = false
     elseif isempty(methods(f)) && isa(f, DataType) && isabstracttype(f)
         print(io, "no constructors have been defined for ", f)
     elseif isempty(methods(f)) && !isa(f, Function) && !isa(f, Type)

base/reduce.jl

@@ -316,10 +316,11 @@ pairwise_blocksize(::typeof(abs2), ::typeof(+)) = 4096
 
 
 # handling empty arrays
-_empty_reduce_error() = throw(ArgumentError("reducing over an empty collection is not allowed"))
-_empty_reduce_error(@nospecialize(f), @nospecialize(T::Type)) = throw(ArgumentError("""
-    reducing with $f over an empty collection of element type $T is not allowed.
-    You may be able to prevent this error by supplying an `init` value to the reducer."""))
+_empty_reduce_error() = throw(ArgumentError("reducing over an empty collection is not allowed; consider supplying `init` to the reducer"))
+reduce_empty(f, T) = _empty_reduce_error()
+mapreduce_empty(f, op, T) = _empty_reduce_error()
+reduce_empty(f, ::Type{Union{}}, splat...) = _empty_reduce_error()
+mapreduce_empty(f, op, ::Type{Union{}}, splat...) = _empty_reduce_error()
 
 """
     Base.reduce_empty(op, T)
@@ -339,20 +340,16 @@ is generally ambiguous, and especially so when the element type is unknown).
 
 As an alternative, consider supplying an `init` value to the reducer.
 """
-reduce_empty(::typeof(+), ::Type{Union{}}) = _empty_reduce_error(+, Union{})
 reduce_empty(::typeof(+), ::Type{T}) where {T} = zero(T)
 reduce_empty(::typeof(+), ::Type{Bool}) = zero(Int)
-reduce_empty(::typeof(*), ::Type{Union{}}) = _empty_reduce_error(*, Union{})
 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(add_sum), ::Type{Union{}}) = _empty_reduce_error(add_sum, Union{})
 reduce_empty(::typeof(add_sum), ::Type{T}) where {T} = reduce_empty(+, T)
 reduce_empty(::typeof(add_sum), ::Type{T}) where {T<:SmallSigned}  = zero(Int)
 reduce_empty(::typeof(add_sum), ::Type{T}) where {T<:SmallUnsigned} = zero(UInt)
-reduce_empty(::typeof(mul_prod), ::Type{Union{}}) = _empty_reduce_error(mul_prod, Union{})
 reduce_empty(::typeof(mul_prod), ::Type{T}) where {T} = reduce_empty(*, T)
 reduce_empty(::typeof(mul_prod), ::Type{T}) where {T<:SmallSigned}  = one(Int)
 reduce_empty(::typeof(mul_prod), ::Type{T}) where {T<:SmallUnsigned} = one(UInt)
@@ -753,7 +750,7 @@ julia> maximum([1,2,3])
 3
 
 julia> maximum(())
-ERROR: MethodError: reducing over an empty collection is not allowed; consider supplying `init` to the reducer
+ERROR: ArgumentError: reducing over an empty collection is not allowed; consider supplying `init` to the reducer
 Stacktrace:
 [...]
 
@@ -785,7 +782,7 @@ julia> minimum([1,2,3])
 1
 
 julia> minimum([])
-ERROR: MethodError: reducing over an empty collection is not allowed; consider supplying `init` to the reducer
+ERROR: ArgumentError: reducing over an empty collection is not allowed; consider supplying `init` to the reducer
 Stacktrace:
 [...]
 

test/reduce.jl

@@ -53,8 +53,8 @@ end
 @test reduce(max, [8 6 7 5 3 0 9]) == 9
 @test reduce(+, 1:5; init=1000) == (1000 + 1 + 2 + 3 + 4 + 5)
 @test reduce(+, 1) == 1
-@test_throws "reducing with * over an empty collection of element type Union{} is not allowed" reduce(*, ())
-@test_throws "reducing with * over an empty collection of element type Union{} is not allowed" reduce(*, Union{}[])
+@test_throws "reducing over an empty collection is not allowed" reduce(*, ())
+@test_throws "reducing over an empty collection is not allowed" reduce(*, Union{}[])
 
 # mapreduce
 @test mapreduce(-, +, [-10 -9 -3]) == ((10 + 9) + 3)
@@ -91,8 +91,7 @@ end
 @test mapreduce(abs2, *, Float64[]) === 1.0
 @test mapreduce(abs2, max, Float64[]) === 0.0
 @test mapreduce(abs, max, Float64[]) === 0.0
-@test_throws ["reducing over an empty collection is not allowed",
-              "consider supplying `init`"] 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[])
 
@@ -144,9 +143,8 @@ fz = float(z)
 @test sum(z) === 136
 @test sum(fz) === 136.0
 
-@test_throws "reducing with add_sum over an empty collection of element type Union{} is not allowed" sum(Union{}[])
-@test_throws ["reducing over an empty collection is not allowed",
-              "consider supplying `init`"] sum(sin, Int[])
+@test_throws "reducing over an empty collection is not allowed" sum(Union{}[])
+@test_throws "reducing over an empty collection is not allowed" sum(sin, Int[])
 @test sum(sin, 3) == sin(3.0)
 @test sum(sin, [3]) == sin(3.0)
 a = sum(sin, z)