fix: correct return tuple for Enzyme's reverse autodiff

Author: gdalle

src/Enzyme.jl

@@ -496,12 +496,11 @@ function overload_autodiff(
     func2.operation = MLIR.API.MlirOperation(C_NULL)
 
     if reverse
-        resv = if EnzymeCore.needs_primal(CMode)
-            result
+        if EnzymeCore.needs_primal(CMode)
+            return ((restup...,), result)
         else
-            nothing
+            return ((restup...,),)
         end
-        return ((restup...,), resv)
     else
         if EnzymeCore.needs_primal(CMode)
             if CMode <: ForwardMode && !(A <: Const)

test/autodiff.jl

@@ -407,3 +407,12 @@ end
     @test results_fd[2].y ≈ results_enz[2].y
     @test results_fd[3] ≈ results_enz[3]
 end
+
+@testset "Correct return tuple" begin
+    # issue 1875
+    x = ones(2)
+    xr = Reactant.to_rarray(x)
+    res = autodiff(Reverse, sum, Duplicated(x, zero(x)))
+    res_reactant = @jit autodiff(Reverse, sum, Duplicated(xr, zero(xr)))
+    @test length(res) == length(res_reactant)
+end