Only define rules for Julia 1.9 onwards

Author: Simone Carlo Surace

src/rulesets/LinearAlgebra/dense.jl

@@ -399,46 +399,48 @@ end
 ##### `kron`
 #####
 
-function frule((_, Δx, Δy), ::typeof(kron), x::AbstractVecOrMat{<:Number}, y::AbstractVecOrMat{<:Number})
-    return kron(x, y), kron(Δx, y) + kron(x, Δy)
-end
+@static if VERSION ≥ v"1.9.0"
+    function frule((_, Δx, Δy), ::typeof(kron), x::AbstractVecOrMat{<:Number}, y::AbstractVecOrMat{<:Number})
+        return kron(x, y), kron(Δx, y) + kron(x, Δy)
+    end
 
-function rrule(::typeof(kron), x::AbstractVector{<:Number}, y::AbstractVector{<:Number})
-    function kron_pullback(z̄)
-        dz = reshape(unthunk(z̄), length(y), length(x))
-        x̄ = @thunk conj.(dz' * y)
-        ȳ = @thunk dz * conj.(x)
-        return NoTangent(), x̄, ȳ
+    function rrule(::typeof(kron), x::AbstractVector{<:Number}, y::AbstractVector{<:Number})
+        function kron_pullback(z̄)
+            dz = reshape(unthunk(z̄), length(y), length(x))
+            x̄ = @thunk conj.(dz' * y)
+            ȳ = @thunk dz * conj.(x)
+            return NoTangent(), x̄, ȳ
+        end
+        return kron(x, y), kron_pullback
     end
-    return kron(x, y), kron_pullback
-end
 
-function rrule(::typeof(kron), x::AbstractMatrix{<:Number}, y::AbstractVector{<:Number})
-    function kron_pullback(z̄)
-        dz = reshape(unthunk(z̄), length(y), size(x)...)
-        x̄ = @thunk conj.(dot.(eachslice(dz; dims = (2, 3)), Ref(y)))
-        ȳ = @thunk conj.(dot.(eachslice(dz; dims = 1), Ref(x)))
-        return NoTangent(), x̄, ȳ
+    function rrule(::typeof(kron), x::AbstractMatrix{<:Number}, y::AbstractVector{<:Number})
+        function kron_pullback(z̄)
+            dz = reshape(unthunk(z̄), length(y), size(x)...)
+            x̄ = @thunk conj.(dot.(eachslice(dz; dims = (2, 3)), Ref(y)))
+            ȳ = @thunk conj.(dot.(eachslice(dz; dims = 1), Ref(x)))
+            return NoTangent(), x̄, ȳ
+        end
+        return kron(x, y), kron_pullback
     end
-    return kron(x, y), kron_pullback
-end
 
-function rrule(::typeof(kron), x::AbstractVector{<:Number}, y::AbstractMatrix{<:Number})
-    function kron_pullback(z̄)
-        dz = reshape(unthunk(z̄), size(y, 1), length(x), size(y, 2))
-        x̄ = @thunk conj.(dot.(eachslice(dz; dims = 2), Ref(y)))
-        ȳ = @thunk conj.(dot.(eachslice(dz; dims = (1, 3)), Ref(x)))
-        return NoTangent(), x̄, ȳ
+    function rrule(::typeof(kron), x::AbstractVector{<:Number}, y::AbstractMatrix{<:Number})
+        function kron_pullback(z̄)
+            dz = reshape(unthunk(z̄), size(y, 1), length(x), size(y, 2))
+            x̄ = @thunk conj.(dot.(eachslice(dz; dims = 2), Ref(y)))
+            ȳ = @thunk conj.(dot.(eachslice(dz; dims = (1, 3)), Ref(x)))
+            return NoTangent(), x̄, ȳ
+        end
+        return kron(x, y), kron_pullback
     end
-    return kron(x, y), kron_pullback
-end
 
-function rrule(::typeof(kron), x::AbstractMatrix{<:Number}, y::AbstractMatrix{<:Number})
-    function kron_pullback(z̄)
-        dz = reshape(unthunk(z̄), size(y, 1), size(x, 1), size(y, 2), size(x, 2))
-        x̄ = @thunk conj.(dot.(eachslice(dz, dims = (2, 4)), Ref(y)))
-        ȳ = @thunk conj.(dot.(eachslice(dz; dims = (1, 3)), Ref(x)))
-        return NoTangent(), x̄, ȳ
+    function rrule(::typeof(kron), x::AbstractMatrix{<:Number}, y::AbstractMatrix{<:Number})
+        function kron_pullback(z̄)
+            dz = reshape(unthunk(z̄), size(y, 1), size(x, 1), size(y, 2), size(x, 2))
+            x̄ = @thunk conj.(dot.(eachslice(dz, dims = (2, 4)), Ref(y)))
+            ȳ = @thunk conj.(dot.(eachslice(dz; dims = (1, 3)), Ref(x)))
+            return NoTangent(), x̄, ȳ
+        end
+        return kron(x, y), kron_pullback
     end
-    return kron(x, y), kron_pullback
 end