use Requires.jl for julia versions <1.9

Author: migrosser

Project.toml

@@ -9,6 +9,7 @@ LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 Printf = "de0858da-6303-5e67-8744-51eddeeeb8d7"
 SparseArrays = "2f01184e-e22b-5df5-ae63-d93ebab69eaf"
 TimerOutputs = "a759f4b9-e2f1-59dc-863e-4aeb61b1ea8f"
+Requires = "ae029012-a4dd-5104-9daa-d747884805df"
 
 [weakdeps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"

ext/LinearOperatorsChainRulesCoreExt.jl

@@ -1,7 +1,7 @@
 module LinearOperatorsChainRulesCoreExt
 
 using LinearOperators
-import ChainRulesCore
+isdefined(Base, :get_extension) ? (import ChainRulesCore) : (import ..ChainRulesCore)
 
 function ChainRulesCore.frule((_, Δx, _), ::typeof(*), op::AbstractLinearOperator{T}, x::AbstractVector{S}) where {T, S}
   y = op*x

src/LinearOperators.jl

@@ -28,4 +28,17 @@ include("TimedOperators.jl")
 include("utilities.jl")
 include("deprecated.jl")
 
+# lazy loading of chainrules for Julia < 1.9
+@static if !isdefined(Base, :get_extension)
+  import Requires
+end
+   
+@static if !isdefined(Base, :get_extension)
+  function __init__()
+    Requires.@require ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4" begin
+      include("../ext/LinearOperatorsChainRulesCoreExt.jl")
+    end
+  end
+end
+
 end # module