fix deprecated call, add test

Author: thevolatilebit

src/forwarddiff.jl

@@ -63,10 +63,12 @@ function (dec::DualEltypeChecker)(::Val{Y}) where {Y}
     anyeltypedual(getproperty(dec.x, Y), dec.counter)
 end
 
-# use `getfield` for `Pairs`, see https://github.com/JuliaLang/julia/pull/39448
-function (dec::DualEltypeChecker{<:Base.Pairs})(::Val{Y}) where {Y}
-    isdefined(dec.x, Y) || return Any
-    anyeltypedual(getfield(dec.x, Y), dec.counter)
+# use `getfield` on `Pairs`, see https://github.com/JuliaLang/julia/pull/39448
+if VERSION >= v"1.7"
+    function (dec::DualEltypeChecker{<:Base.Pairs})(::Val{Y}) where {Y}
+        isdefined(dec.x, Y) || return Any
+        anyeltypedual(getfield(dec.x, Y), dec.counter)
+    end
 end
 
 # Untyped dispatch: catch composite types, check all of their fields

test/forwarddiff_dual_detection.jl

@@ -222,3 +222,7 @@ for p in p_possibilities_configs_not_inferred
     u0 = ForwardDiff.Dual(2.0)
     @test DiffEqBase.promote_u0(u0, p, t0) isa ForwardDiff.Dual
 end
+
+# use `getfield` on `Pairs`, see https://github.com/JuliaLang/julia/pull/39448
+VERSION >= v"1.7" && 
+           @test_nowarn DiffEqBase.DualEltypeChecker(pairs((;)), 0)(Val(:data))