okay understood why it fails.

Author: lrnv

test/chainrules.jl

@@ -205,11 +205,11 @@
             # Forward da
             _, Δx_inv_da = frule((NoTangent(), 1.0, 0.0, 0.0), beta_inc_inv, a, b, p)
             _, Δp_from_da = frule((NoTangent(), 1.0, 0.0, Δx_inv_da[1]), beta_inc, a, b, x_inv)
-            @test iszero(Δp_from_da[1])
+            @test isapprox(Δp_from_da[1], 0.0, rtol=1e-9, atol=1e-15)
             # Forward db
             _, Δx_inv_db = frule((NoTangent(), 0.0, 1.0, 0.0), beta_inc_inv, a, b, p)
             _, Δp_from_db = frule((NoTangent(), 0.0, 1.0, Δx_inv_db[1]), beta_inc, a, b, x_inv)
-            @test isapprox(Δp_from_db[1], 0.0, rtol=1e-9)
+            @test isapprox(Δp_from_db[1], 0.0, rtol=1e-9, atol=1e-15)
 
             # Reverse-mode chain for composition: pullback through f then g
             # Pullback of f at (a,b,x_inv)