mr_empty specialization for promote_sys_size

Author: TotalVerb

base/reduce.jl

@@ -242,6 +242,12 @@ mr_empty(::typeof(abs2), op::typeof(max), T) = mr_empty(abs2, scalarmax, T)
 mr_empty(f, op::typeof(&), T) = true
 mr_empty(f, op::typeof(|), T) = false
 
+# Allow mr_empty to “see through” promote_sys_size
+let ComposedFunction = typename(typeof(identity ∘ identity)).wrapper
+    global mr_empty(f::ComposedFunction{typeof(promote_sys_size)}, op, T) =
+        promote_sys_size(mr_empty(f.g, op, T))
+end
+
 mr_empty_iter(f, op, itr, ::HasEltype) = mr_empty(f, op, eltype(itr))
 mr_empty_iter(f, op::typeof(&), itr, ::EltypeUnknown) = true
 mr_empty_iter(f, op::typeof(|), itr, ::EltypeUnknown) = false

test/reduce.jl

@@ -144,6 +144,14 @@ end
 @test sum_kbn([-0.0]) === -0.0
 @test sum_kbn([-0.0,-0.0]) === -0.0
 
+# check sum(abs, ...) for support of empty collections
+@testset "sum(abs, [])" begin
+    @test @inferred(sum(abs, Float64[])) === 0.0
+    @test @inferred(sum(abs, Int[])) === 0
+    @test @inferred(sum(abs, Set{Int}())) === 0
+    @test_throws MethodError sum(abs, Any[])
+end
+
 # prod
 
 @test prod(Int[]) === 1