Merge pull request #20234 from JuliaLang/mh/broadcast_identity

Add `broadcast!(identity, ::Array, ::Array)` specialization

Author: tkelman

base/broadcast.jl

@@ -15,6 +15,8 @@ typealias ScalarType Union{Type{Any}, Type{Nullable}}
 # fallbacks for some special cases
 @inline broadcast(f, x::Number...) = f(x...)
 @inline broadcast{N}(f, t::NTuple{N,Any}, ts::Vararg{NTuple{N,Any}}) = map(f, t, ts...)
+broadcast!{T,S,N}(::typeof(identity), x::Array{T,N}, y::Array{S,N}) =
+    size(x) == size(y) ? copy!(x, y) : broadcast_c!(identity, Array, Array, x, y)
 
 # special cases for "X .= ..." (broadcast!) assignments
 broadcast!(::typeof(identity), X::AbstractArray, x::Number) = fill!(X, x)

test/broadcast.jl

@@ -449,3 +449,12 @@ end
     @test broadcast(==, [1], AbstractArray) == BitArray([false])
     @test broadcast(==, 1, AbstractArray) == false
 end
+
+# Test that broadcasting identity where the input and output Array shapes do not match
+# yields the correct result, not merely a partial copy. See pull request #19895 for discussion.
+let N = 5
+    @test iszero(ones(N, N) .= zeros(N, N))
+    @test iszero(ones(N, N) .= zeros(N, 1))
+    @test iszero(ones(N, N) .= zeros(1, N))
+    @test iszero(ones(N, N) .= zeros(1, 1))
+end