leave D(foo(t)) untouched if foo is not a known function.

Author: shashi

src/differentials.jl

@@ -101,9 +101,11 @@ function expand_derivatives(O::Term, simplify=true; occurances=nothing)
             x = if _iszero(t2)
                 t2
             elseif _isone(t2)
-                derivative(o, i)
+                d = derivative(o, i)
+                d isa NoDeriv ? D(o) : d
             else
                 t1 = derivative(o, i)
+                t1 = t1 isa NoDeriv ? D(o) : t1
                 make_operation(*, [t1, t2])
             end
 
@@ -181,6 +183,11 @@ sin(x())
 derivative(O::Term, idx) = derivative(O.op, (O.args...,), Val(idx))
 derivative(O::Any, ::Any) = 0
 
+# Indicate that no derivative is defined.
+struct NoDeriv
+end
+derivative(f, args::Tuple, v::Val) = NoDeriv()
+
 # Pre-defined derivatives
 import DiffRules
 for (modu, fun, arity) ∈ DiffRules.diffrules()

test/direct.jl

@@ -167,3 +167,14 @@ test_worldage()
 @test_nowarn muladd(x, y, 0)
 @test promote(x, 0) == (x, identity(0))
 @test_nowarn [x, y, z]'
+
+let
+    @register foo(x)
+    @variables t
+    @derivatives D'~t
+
+
+    @test isequal(expand_derivatives(D(foo(t))), D(foo(t)))
+    @test isequal(expand_derivatives(D(sin(t) * foo(t))), cos(t) * foo(t) + sin(t) * D(foo(t)))
+
+end