Merge pull request #2926 from stan-dev/fix/stan-print-forward-decl

Add forward declaration to stan_print.hpp

Author: WardBrian

stan/math/prim/fun/stan_print.hpp

@@ -52,6 +52,10 @@ void stan_print(std::ostream* o, const EigMat& x) {
   *o << ']';
 }
 
+// forward decl to allow the next two overloads to call each other
+template <typename T, require_tuple_t<T>* = nullptr>
+void stan_print(std::ostream* o, const T& x);
+
 template <typename T, require_std_vector_t<T>* = nullptr>
 void stan_print(std::ostream* o, const T& x) {
   *o << '[';
@@ -64,7 +68,7 @@ void stan_print(std::ostream* o, const T& x) {
   *o << ']';
 }
 
-template <typename T, require_tuple_t<T>* = nullptr>
+template <typename T, require_tuple_t<T>*>
 void stan_print(std::ostream* o, const T& x) {
   *o << '(';
   constexpr auto tuple_size = std::tuple_size<std::decay_t<T>>::value;

test/unit/math/prim/fun/stan_print_test.cpp

@@ -11,6 +11,9 @@ TEST(MathPrim, basic_print) {
 
   std::tuple<Eigen::VectorXd, int, std::vector<double>> tup(v, i, a);
 
+  std::vector<std::tuple<Eigen::VectorXd, int, std::vector<double>>> arr_tup{
+      tup, tup};
+
   {
     std::stringstream s;
     stan::math::stan_print(&s, i);
@@ -52,6 +55,11 @@ TEST(MathPrim, basic_print) {
     stan::math::stan_print(&s, tup);
     EXPECT_TRUE(s.str().find("([1],1,[1])") != std::string::npos);
   }
+  {
+    std::stringstream s;
+    stan::math::stan_print(&s, arr_tup);
+    EXPECT_TRUE(s.str().find("[([1],1,[1])") != std::string::npos);
+  }
 }
 
 TEST(MathPrim, basic_expressions) {