From 01d9b8155d6dcf806b04353ee71758585cbd08ae Mon Sep 17 00:00:00 2001
From: Simon Byrne <simonbyrne@gmail.com>
Date: Thu, 25 Apr 2019 17:02:23 -0700
Subject: [PATCH] Remove definitions for `reduce` over `&`/`|` over empty
 collections of unknown eltype.

Fixes #31635/#31837.
---
 NEWS.md        |  1 +
 base/reduce.jl | 17 ++++++-----------
 test/syntax.jl |  2 +-
 3 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index e5d3619986d087..9c24f581104e63 100644
--- a/NEWS.md
+++ b/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
 -----------------------
diff --git a/base/reduce.jl b/base/reduce.jl
index 4113f9a7368434..02a0f654f0a0d6 100644
--- a/base/reduce.jl
+++ b/base/reduce.jl
@@ -54,7 +54,7 @@ end
 function mapfoldl_impl(f, op, nt::NamedTuple{()}, itr)
     y = iterate(itr)
     if y === nothing
-        return Base.mapreduce_empty_iter(f, op, itr, IteratorEltype(itr))
+        return Base.mapreduce_empty(f, op, eltype(itr))
     end
     (x, i) = y
     init = mapreduce_first(f, op, x)
@@ -110,7 +110,7 @@ end
 
 function mapfoldr_impl(f, op, ::NamedTuple{()}, itr, i::Integer)
     if isempty(itr)
-        return Base.mapreduce_empty_iter(f, op, itr, IteratorEltype(itr))
+        return Base.mapreduce_empty(f, op, eltype(itr))
     end
     return mapfoldr_impl(f, op, (init=mapreduce_first(f, op, itr[i]),), itr, i-1)
 end
@@ -228,9 +228,9 @@ with reduction `op` over an empty array with element type of `T`.
 If not defined, this will throw an `ArgumentError`.
 """
 reduce_empty(op, T) = _empty_reduce_error()
-reduce_empty(::typeof(+), T) = zero(T)
+reduce_empty(::typeof(+), ::Type{<:Number}) = zero(T)
 reduce_empty(::typeof(+), ::Type{Bool}) = zero(Int)
-reduce_empty(::typeof(*), T) = one(T)
+reduce_empty(::typeof(*), ::Type{<:Number}) = one(T)
 reduce_empty(::typeof(*), ::Type{<:AbstractChar}) = ""
 reduce_empty(::typeof(&), ::Type{Bool}) = true
 reduce_empty(::typeof(|), ::Type{Bool}) = false
@@ -256,13 +256,8 @@ 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_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(::typeof(abs),  ::typeof(max), T) = abs(zero(T))
+mapreduce_empty(::typeof(abs2), ::typeof(max), T) = abs2(zero(T))
 
 # handling of single-element iterators
 """
diff --git a/test/syntax.jl b/test/syntax.jl
index 85951185f4a57d..2f2291197e8dcf 100644
--- a/test/syntax.jl
+++ b/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])