From 256814a350abb01b9a1f76016d888808fb256404 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 | 10 ++++------
 test/reduce.jl |  2 +-
 test/syntax.jl |  2 +-
 4 files changed, 7 insertions(+), 8 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..230d9d2c8b0dea 100644
--- a/base/reduce.jl
+++ b/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
 """
diff --git a/test/reduce.jl b/test/reduce.jl
index cc4ce66d2d7d43..2daa9903576ea6 100644
--- a/test/reduce.jl
+++ b/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
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])