diff --git a/test/issues/25.lean b/test/issues/25.lean
new file mode 100644
index 00000000..0361dad3
--- /dev/null
+++ b/test/issues/25.lean
@@ -0,0 +1,28 @@
+import SciLean
+
+open SciLean
+
+variable 
+  (K : Type _) [IsROrC K]
+  {X : Type _} [SemiInnerProductSpace K X]
+  {Y : Type _} [SemiInnerProductSpace K Y]
+  {Z : Type _} [SemiInnerProductSpace K Z]
+
+example
+  (f : Y → Z) (g : X → Y) 
+  (hf : HasAdjDiff K f) (hg : HasAdjDiff K g)
+  : revDerivUpdate K (fun x : X => f (g x))
+    = 
+    fun x =>
+      let ydg := revDerivUpdate K g x
+      let zdf := revDerivUpdate K (fun x' => f (ydg.1 + semiAdjoint K (ydg.2 · 1 0) (x' - x))) x
+      zdf := 
+by
+  have ⟨_,_⟩ := hf
+  have ⟨_,_⟩ := hg
+  unfold revDerivUpdate
+  funext _; 
+  -- failed to synthesize
+  --   SemiInnerProductSpace K (X → X)
+  -- ftrans 
+  sorry