Remove definitions for reduce over &/| over empty collections o…

…f unknown eltype.

Fixes #31635/#31837.

NEWS.md

@@ -8,6 +8,7 @@ New language features
 Language changes
 ----------------
 
+* `reduce`/`mapreduce` using `&` and `|` as an operator over empty collections of unknown element type will throw an error instead of returning `true`/`false` ([#31635], [#31837]).
 
 Multi-threading changes
 -----------------------

base/reduce.jl

@@ -255,14 +255,12 @@ mapreduce_empty(f, op, T) = _empty_reduce_error()
 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(abs2), ::typeof(max), T) = abs2(zero(T))
 
 mapreduce_empty_iter(f, op, itr, ::HasEltype) = mapreduce_empty(f, op, eltype(itr))
-mapreduce_empty_iter(f, op::typeof(&), itr, ::EltypeUnknown) = true
-mapreduce_empty_iter(f, op::typeof(|), itr, ::EltypeUnknown) = false
-mapreduce_empty_iter(f, op, itr, ::EltypeUnknown) = _empty_reduce_error()
+mapreduce_empty_iter(f, op, itr, ::EltypeUnknown) = mapreduce_empty(f, op, Any)
+
 
 # handling of single-element iterators
 """

test/reduce.jl

@@ -489,7 +489,7 @@ end
 # issue #18695
 test18695(r) = sum( t^2 for t in r )
 @test @inferred(test18695([1.0,2.0,3.0,4.0])) == 30.0
-@test_throws ArgumentError test18695(Any[])
+@test_throws MethodError test18695(Any[])
 
 # issue #21107
 @test foldr(-,2:2) == 2

test/syntax.jl

@@ -287,7 +287,7 @@ call2(f,x,y) = f(x,y)
 @test (call2(42,1) do x,y; x+y+1 end) == 44
 
 # definitions using comparison syntax
-let a⊂b = reduce(&, x ∈ b for x in a) && length(b)>length(a)
+let a⊂b = all(x ∈ b for x in a) && length(b)>length(a)
     @test [1,2] ⊂ [1,2,3,4]
     @test !([1,2] ⊂ [1,3,4])
     @test !([1,2] ⊂ [1,2])