Fix broadcast for mis-matched arrays (#159)

src/broadcast.jl

@@ -89,6 +89,31 @@ function broadcast_mutability(x, op, args::Vararg{Any,N}) where {N}
     return broadcast_mutability(typeof(x), op, typeof.(args)...)
 end
 
+_checked_size(s, x::AbstractArray) = size(x) == s
+_checked_size(::Any, ::Any) = true
+_checked_size(::Any, ::Tuple{}) = true
+function _checked_size(s, x::Tuple)
+    return _checked_size(s, x[1]) && _checked_size(s, Base.tail(x))
+end
+
+# This method is a slightly tricky one:
+#
+# If the elements in the broadcast are different sized arrays, weird things can
+# happen during broadcasting since we'll either need to return a different size
+# to `x`, or multiple copies of an argument will be used for different parts of
+# `x`. To simplify, let's just return `IsNotMutable` if the sizes are different,
+# which will be slower but correct.
+function broadcast_mutability(
+    x::AbstractArray,
+    op,
+    args::Vararg{Any,N},
+) where {N}
+    if !_checked_size(size(x), args)::Bool
+        return IsNotMutable()
+    end
+    return broadcast_mutability(typeof(x), op, typeof.(args)...)
+end
+
 broadcast_mutability(::Type) = IsNotMutable()
 
 """

test/broadcast.jl

@@ -19,6 +19,7 @@ const MA = MutableArithmetics
     alloc_test(() -> MA.broadcast!!(+, a, b), 0)
     alloc_test(() -> MA.broadcast!!(+, a, c), 0)
 end
+
 @testset "BigInt" begin
     x = BigInt(1)
     y = BigInt(2)
@@ -34,3 +35,13 @@ end
     alloc_test(() -> MA.broadcast!!(+, a, b), 30 * sizeof(Int))
     alloc_test(() -> MA.broadcast!!(+, a, c), 0)
 end
+
+@testset "broadcast_issue_158" begin
+    x, y = BigInt[2 3], BigInt[2 3; 3 4]
+    @test MA.@rewrite(x .+ y) == x .+ y
+    @test MA.@rewrite(x .- y) == x .- y
+    @test MA.@rewrite(y .+ x) == y .+ x
+    @test MA.@rewrite(y .- x) == y .- x
+    @test MA.@rewrite(y .* x) == y .* x
+    @test MA.@rewrite(x .* y) == x .* y
+end