diff --git a/base/abstractarray.jl b/base/abstractarray.jl
index e97359cb87fcf..7e51cfa2b19d2 100644
--- a/base/abstractarray.jl
+++ b/base/abstractarray.jl
@@ -891,13 +891,12 @@ See also [`copyto!`](@ref).
     is available from the `Future` standard library as `Future.copy!`.
 """
 function copy!(dst::AbstractVector, src::AbstractVector)
+    firstindex(dst) == firstindex(src) || throw(ArgumentError(
+        "vectors must have the same offset for copy! (consider using `copyto!`)"))
     if length(dst) != length(src)
         resize!(dst, length(src))
     end
-    for i in eachindex(dst, src)
-        @inbounds dst[i] = src[i]
-    end
-    dst
+    copyto!(dst, src)
 end
 
 function copy!(dst::AbstractArray, src::AbstractArray)
@@ -1108,8 +1107,9 @@ function copyto!(dest::AbstractArray, dstart::Integer,
     destinds, srcinds = LinearIndices(dest), LinearIndices(src)
     (checkbounds(Bool, destinds, dstart) && checkbounds(Bool, destinds, dstart+n-1)) || throw(BoundsError(dest, dstart:dstart+n-1))
     (checkbounds(Bool, srcinds, sstart)  && checkbounds(Bool, srcinds, sstart+n-1))  || throw(BoundsError(src,  sstart:sstart+n-1))
-    @inbounds for i = 0:(n-1)
-        dest[dstart+i] = src[sstart+i]
+    src′ = unalias(dest, src)
+    @inbounds for i = 0:n-1
+        dest[dstart+i] = src′[sstart+i]
     end
     return dest
 end
@@ -1131,11 +1131,12 @@ function copyto!(B::AbstractVecOrMat{R}, ir_dest::AbstractRange{Int}, jr_dest::A
     end
     @boundscheck checkbounds(B, ir_dest, jr_dest)
     @boundscheck checkbounds(A, ir_src, jr_src)
+    A′ = unalias(B, A)
     jdest = first(jr_dest)
     for jsrc in jr_src
         idest = first(ir_dest)
         for isrc in ir_src
-            @inbounds B[idest,jdest] = A[isrc,jsrc]
+            @inbounds B[idest,jdest] = A′[isrc,jsrc]
             idest += step(ir_dest)
         end
         jdest += step(jr_dest)
diff --git a/base/bitarray.jl b/base/bitarray.jl
index 71d83b5b58f56..095acf6a519e3 100644
--- a/base/bitarray.jl
+++ b/base/bitarray.jl
@@ -458,10 +458,11 @@ function unsafe_copyto!(dest::BitArray, doffs::Integer, src::Union{BitArray,Arra
     return dest
 end
 
-copyto!(dest::BitArray, doffs::Integer, src::Array, soffs::Integer, n::Integer) =
+copyto!(dest::BitArray, doffs::Integer, src::Union{BitArray,Array}, soffs::Integer, n::Integer) =
     _copyto_int!(dest, Int(doffs), src, Int(soffs), Int(n))
-function _copyto_int!(dest::BitArray, doffs::Int, src::Array, soffs::Int, n::Int)
+function _copyto_int!(dest::BitArray, doffs::Int, src::Union{BitArray,Array}, soffs::Int, n::Int)
     n == 0 && return dest
+    n < 0 && throw(ArgumentError("Number of elements to copy must be nonnegative."))
     soffs < 1 && throw(BoundsError(src, soffs))
     doffs < 1 && throw(BoundsError(dest, doffs))
     soffs+n-1 > length(src) && throw(BoundsError(src, length(src)+1))
diff --git a/test/bitarray.jl b/test/bitarray.jl
index 3a528d4391a82..c1c596dc5d7d6 100644
--- a/test/bitarray.jl
+++ b/test/bitarray.jl
@@ -15,12 +15,11 @@ bitcheck(x) = true
 bcast_setindex!(b, x, I...) = (b[I...] .= x; b)
 
 function check_bitop_call(ret_type, func, args...; kwargs...)
-    r1 = func(args...; kwargs...)
     r2 = func(map(x->(isa(x, BitArray) ? Array(x) : x), args)...; kwargs...)
-    ret_type ≢ nothing && !isa(r1, ret_type) && @show ret_type, typeof(r1)
-    ret_type ≢ nothing && @test isa(r1, ret_type)
+    r1 = func(args...; kwargs...)
+    ret_type ≢ nothing && (@test isa(r1, ret_type) || @show ret_type, typeof(r1))
     @test tc(r1, r2)
-    @test isequal(r1, ret_type ≡ nothing ? r2 : r2)
+    @test isequal(r1, r2)
     @test bitcheck(r1)
 end
 macro check_bit_operation(ex, ret_type)
@@ -518,12 +517,14 @@ timesofar("constructors")
             end
         end
 
+        self_copyto!(a, n1, n2, l) = copyto!(a, n1, a, n2, l)
         for p1 = [rand(1:v1) 1 63 64 65 191 192 193]
             for p2 = [rand(1:v1) 1 63 64 65 191 192 193]
                 for n = 0 : min(v1 - p1 + 1, v1 - p2 + 1)
                     b1 = bitrand(v1)
                     b2 = bitrand(v1)
                     @check_bit_operation copyto!(b1, p1, b2, p2, n) BitVector
+                    @check_bit_operation self_copyto!(b1, p1, p2, n) BitVector
                 end
             end
         end
diff --git a/test/copy.jl b/test/copy.jl
index 04fda36728e62..c4fcff40f2b3f 100644
--- a/test/copy.jl
+++ b/test/copy.jl
@@ -245,3 +245,10 @@ end
 @testset "deepcopy_internal arrays" begin
     @test (@inferred Base.deepcopy_internal(zeros(), IdDict())) == zeros()
 end
+
+@testset "`copyto!`'s unaliasing" begin
+    a = view([1:3;], :)
+    @test copyto!(a, 2, a, 1, 2) == [1;1:2;]
+    a = [1:3;]
+    @test copyto!(a, 2:3, 1:1, a, 1:2, 1:1) == [1;1:2;]
+end