diff --git a/base/reducedim.jl b/base/reducedim.jl
index c1c58ccdfefed..aa3408b036587 100644
--- a/base/reducedim.jl
+++ b/base/reducedim.jl
@@ -528,6 +528,9 @@ Sum elements of `A` over the singleton dimensions of `r`, and write results to `
 Note that since the sum! function is intended to operate without making any allocations,
 the target should not alias with the source.
 
+!!! warning
+   Incorrect result may occur when `r` is alias or `view` of `A`.
+
 # Examples
 ```jldoctest
 julia> A = [1 2; 3 4]
@@ -601,6 +604,9 @@ prod(f, A::AbstractArray; dims)
 
 Multiply elements of `A` over the singleton dimensions of `r`, and write results to `r`.
 
+!!! warning
+   Incorrect result may occur when `r` is alias or `view` of `A`.
+
 # Examples
 ```jldoctest
 julia> A = [1 2; 3 4]
@@ -895,6 +901,9 @@ all(::Function, ::AbstractArray; dims)
 
 Test whether all values in `A` along the singleton dimensions of `r` are `true`, and write results to `r`.
 
+!!! warning
+   Incorrect result may occur when `r` is alias or `view` of `A`.
+
 # Examples
 ```jldoctest
 julia> A = [true false; true false]
@@ -968,6 +977,9 @@ any(::Function, ::AbstractArray; dims)
 Test whether any values in `A` along the singleton dimensions of `r` are `true`, and write
 results to `r`.
 
+!!! warning
+   Incorrect result may occur when `r` is alias or `view` of `A`.
+
 # Examples
 ```jldoctest
 julia> A = [true false; true false]
@@ -1019,7 +1031,13 @@ for (fname, op) in [(:sum, :add_sum), (:prod, :mul_prod),
     @eval begin
         $(fname!)(f::Function, r::AbstractArray, A::AbstractArray; init::Bool=true) =
             mapreducedim!($mapf, $(op), initarray!(r, $mapf, $(op), init, A), A)
-        $(fname!)(r::AbstractArray, A::AbstractArray; init::Bool=true) = $(fname!)(identity, r, A; init=init)
+        @inline function $(fname!)(r::AbstractArray, A::AbstractArray; init::Bool=true)
+            if size(r) == size(A) && init
+                copyto!(r, A)
+                return r
+            end
+            $(fname!)(identity, r, A; init=init)
+        end
 
         $(_fname)(A, dims; kw...)    = $(_fname)(identity, A, dims; kw...)
         $(_fname)(f, A, dims; kw...) = mapreduce($mapf, $(op), A; dims=dims, kw...)
diff --git a/test/reducedim.jl b/test/reducedim.jl
index daa0a3fbe1f92..0db60db9543b2 100644
--- a/test/reducedim.jl
+++ b/test/reducedim.jl
@@ -97,6 +97,18 @@ safe_minabs(A::Array{T}, region) where {T} = safe_mapslices(minimum, abs.(A), re
     )
 end
 
+@testset "test in-place reductions with alias" begin
+    a = rand(3)
+    b = copy(a)
+    @test sum!(a, a) == b
+    @test prod!(a, a) == b
+
+    c = rand(Bool, 5)
+    d = copy(c)
+    @test all!(c, c) == d
+    @test any!(c, c) == d
+end
+
 # Combining dims and init
 A = Array{Int}(undef, 0, 3)
 @test_throws "reducing over an empty collection is not allowed" maximum(A; dims=1)