Fix check mutation to return nothing (#145)

Author: gdalle

DifferentiationInterface/docs/src/backends.md

@@ -7,7 +7,7 @@ CollapsedDocStrings = true
 using DifferentiationInterface
 using DifferentiationInterface: backend_string
 import Markdown
-import Enzyme, FastDifferentiation, FiniteDiff, FiniteDifferences, ForwardDiff, PolyesterForwardDiff, ReverseDiff, Tracker, Zygote
+import Diffractor, Enzyme, FastDifferentiation, FiniteDiff, FiniteDifferences, ForwardDiff, PolyesterForwardDiff, ReverseDiff, Tapir, Tracker, Zygote
 
 function all_backends()
     return [

DifferentiationInterface/src/backends.jl

@@ -20,7 +20,10 @@ function check_available(backend::AbstractADType)
     end
 end
 
-square!(y, x) = y .= x .^ 2
+function square!(y, x)
+    y .= x .^ 2
+    return nothing
+end
 
 """
     check_mutation(backend)