Compute discriminant of power series polynomial using substitution.

While other tickets care about making the resultant more flexible, to allow
its operation on polynomials over power series, for the special case of the
discriminant the substitution is certainly a viable and fast alternative.

src/sage/rings/polynomial/polynomial_element.pyx

@@ -4989,11 +4989,24 @@ cdef class Polynomial(CommutativeAlgebraElement):
             sage: E2 = E1.subs(t1=t2, x1=x2, y1=y2)
             sage: det(mu*E1 + E2).discriminant().degrees()
             (24, 12, 12, 8, 8, 8, 8)
+
+        This addresses an issue raised by :trac:`15061`::
+
+            sage: R.<T> = PowerSeriesRing(QQ)
+            sage: F = R([1,1],2)
+            sage: RP.<x> = PolynomialRing(R)
+            sage: P = x^2 - F
+            sage: P.discriminant()
+            4 + 4*T + O(T^2)
         """
+        # Late import to avoid cyclic dependencies:
+        from sage.rings.power_series_ring import is_PowerSeriesRing
         if self.is_zero():
             return self.parent().zero_element()
         poly = self
-        if is_MPolynomialRing(self.parent().base_ring()):
+        base_ring = self.parent().base_ring()
+        if (is_MPolynomialRing(base_ring) or
+            is_PowerSeriesRing(base_ring)):
             # It is often cheaper to compute discriminant of simple
             # multivariate polynomial and substitute the real
             # coefficients into that result (see #16014).