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.

This partly reverts f2dcc44, but our
edges against methods should be better than the ones against
MethodError these days.

base/errorshow.jl

@@ -254,7 +254,6 @@ function showerror(io::IO, ex::MethodError)
         return showerror_ambiguous(io, meth, f, arg_types)
     end
     arg_types_param::SimpleVector = arg_types.parameters
-    show_candidates = true
     print(io, "MethodError: ")
     ft = typeof(f)
     f_is_function = false
@@ -270,9 +269,6 @@ 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)
@@ -346,11 +342,12 @@ function showerror(io::IO, ex::MethodError)
         end
     end
     Experimental.show_error_hints(io, ex, arg_types_param, kwargs)
-    show_candidates && try
+    try
         show_method_candidates(io, ex, kwargs)
     catch ex
         @error "Error showing method candidates, aborted" exception=ex,catch_backtrace()
     end
+    nothing
 end
 
 striptype(::Type{T}) where {T} = T

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)