Why `unable to convert (sin(h(x)), cos(h(x))) to a symbolic expression`?

edit

I had the same problem, I don't know the reason why g(x) = f(h(x)) does not work. But this works (Sage 9.2):

g(x) = [*f(h(x))]

and for g, and g(x) you get:

x ↦ (sin(h(x)),cos(h(x)))
(sin(h(x)),cos(h(x)))

Here, we are basically unpacking and then repacking the array!

Following @slelievre 's comment, here is a possible answer using maps $\mathbb{R} \to \mathbb{R}^2$ and $\mathbb{R}\to\mathbb{R}$:

We define first $f$ as a differentiable map $\mathbb{R} \to \mathbb{R}^2$ :

sage: R.<x> = RealLine()                                                                                      
sage: R2.<X,Y> = EuclideanSpace(name='R^2')                                                                   
sage: f = R.diff_map(R2, (sin(x), cos(x)))                                                                    
sage: f.display()                                                                                             
R --> R^2
   x |--> (X, Y) = (sin(x), cos(x))
sage: f(x)                                                                                                              
Point on the Euclidean plane R^2
sage: f(x).coord()                                                                                                      
(sin(x), cos(x))
sage: f(pi).coord()                                                                                                     
(0, -1)

One can also access to the coordinate functions representing $f$:

sage: fc = f.coord_functions()                                                                                          
sage: fc                                                                                                                
Coordinate functions (sin(x), cos(x)) on the Chart (R, (x,))
sage: fc(x)                                                                                                             
(sin(x), cos(x))
sage: fc(pi)                                                                                                            
(0, -1)

Then we define $H$ as the differentiable map $\mathbb{R}\to\mathbb{R}$ whose coordinate expression is $h(x)$:

sage: H = R.diff_map(R, function('h')(x))                                                                     
sage: H.display()                                                                                             
R --> R
   x |--> h(x)

and we compose $f$ by $H$ by means of the operator *:

sage: g = f * H                                                                                               
sage: g                                                                                                       
Curve in the Euclidean plane R^2
sage: g.display()                                                                                             
R --> R^2
   x |--> (X, Y) = (sin(h(x)), cos(h(x)))

Then

sage: g(x)                                                                                                    
Point on the Euclidean plane R^2
sage: g(x).coord()                                                                                            
(sin(h(x)), cos(h(x)))
sage: g(pi).coord()                                                                                           
(sin(h(pi)), cos(h(pi)))
sage: gc = g.coord_functions(); gc                                                                                      
Coordinate functions (sin(h(x)), cos(h(x))) on the Chart (R, (x,))
sage: gc(pi)                                                                                                            
(sin(h(pi)), cos(h(pi)))

Is this :

sage: def f(*args): return(sin(args[0]),cos(args[0]))
sage: h=function("h")
sage: f(h(x))
(sin(h(x)), cos(h(x)))

what you want ?

A symbolic function returns SR value. A list, a tuple, a vector are not in SR.