fix pow(0, 0) -> undefined partials

Author: Jean-Christophe Ruel

stan/math/rev/fun/pow.hpp

@@ -87,8 +87,8 @@ inline var pow(const Scal1& base, const Scal2& exponent) {
   return make_callback_var(
       std::pow(value_of(base), value_of(exponent)),
       [base, exponent](auto&& vi) mutable {
-        if (value_of(base) == 0.0) {
-          return;  // partials zero, avoids 0 & log(0)
+        if (value_of(base) == 0.0 && value_of(exponent) != 0.0) { 
+          return; // partials zero, avoids 0 & log(0)
         }
         const double vi_mul = vi.adj() * vi.val();
 

test/unit/math/mix/fun/pow_part1_test.cpp

@@ -39,6 +39,7 @@ TEST(mathMixScalFun, pow) {
     return pow(x1, x2);
   };
 
+  stan::test::expect_ad(f, 0.0, 0.0);
   stan::test::expect_ad(f, -0.4, 0.5);
   stan::test::expect_ad(f, 0.5, 0.5);
   stan::test::expect_ad(f, 0.5, 1.0);