17849: adapt SubstituteFunction; add doctest

Author: rwst

src/sage/symbolic/expression.pyx

@@ -4428,6 +4428,17 @@ cdef class Expression(CommutativeRingElement):
             sage: f = foo(x) + 1/foo(pi*y)
             sage: f.substitute_function(foo, bar)
             1/bar(pi*y) + bar(x)
+
+        TESTS:
+
+        Make sure :trac:`17849` is fixed::
+
+            sage: ex = sin(x) + atan2(0,0,hold=True)
+            sage: ex.substitute_function(sin,cos)
+            arctan2(0, 0) + cos(x)
+            sage: ex = sin(x) + hypergeometric([1, 1], [2], -1)
+            sage: ex.substitute_function(sin,cos)
+            cos(x) + hypergeometric((1, 1), (2,), -1)
         """
         from sage.symbolic.expression_conversions import SubstituteFunction
         return SubstituteFunction(self, original, new)()

src/sage/symbolic/expression_conversions.py

@@ -1807,7 +1807,7 @@ def composition(self, ex, operator):
         if operator == self.original:
             return self.new(*map(self, ex.operands()))
         else:
-            return operator(*map(self, ex.operands()))
+            return super(SubstituteFunction, self).composition(ex, operator)
 
     def derivative(self, ex, operator):
         """