Allow size-0 matrices

Author: WardBrian

stan/math/prim/fun/complex_schur_decompose.hpp

@@ -27,7 +27,8 @@ namespace math {
 template <typename M, require_eigen_dense_dynamic_t<M>* = nullptr>
 inline Eigen::Matrix<complex_return_t<scalar_type_t<M>>, -1, -1>
 complex_schur_decompose_u(const M& m) {
-  check_nonzero_size("complex_schur_decompose_u", "m", m);
+  if (m.size() == 0)
+    return m;
   check_square("complex_schur_decompose_u", "m", m);
   using MatType = Eigen::Matrix<scalar_type_t<M>, -1, -1>;
   // copy because ComplexSchur requires Eigen::Matrix type
@@ -50,7 +51,8 @@ complex_schur_decompose_u(const M& m) {
 template <typename M, require_eigen_dense_dynamic_t<M>* = nullptr>
 inline Eigen::Matrix<complex_return_t<scalar_type_t<M>>, -1, -1>
 complex_schur_decompose_t(const M& m) {
-  check_nonzero_size("complex_schur_decompose_t", "m", m);
+  if (m.size() == 0)
+    return m;
   check_square("complex_schur_decompose_t", "m", m);
   using MatType = Eigen::Matrix<scalar_type_t<M>, -1, -1>;
   // copy because ComplexSchur requires Eigen::Matrix type

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

@@ -66,15 +66,15 @@ TEST(mathMixFun, complexSchurDecompose) {
   using ffd_t = stan::math::fvar<fd_t>;
   using fv_t = stan::math::fvar<v_t>;
   using ffv_t = stan::math::fvar<fv_t>;
-  for (const auto& x : stan::test::square_test_matrices(1, 3)) {
+  for (const auto& x : stan::test::square_test_matrices(0, 3)) {
     test_complex_schur_decompose<d_t>(x);
     test_complex_schur_decompose<v_t>(x);
     test_complex_schur_decompose<fd_t>(x);
     test_complex_schur_decompose<ffd_t>(x);
     test_complex_schur_decompose<fv_t>(x);
     test_complex_schur_decompose<ffv_t>(x);
   }
-  for (const auto& x : stan::test::square_test_matrices(1, 3)) {
+  for (const auto& x : stan::test::square_test_matrices(0, 3)) {
     test_complex_schur_decompose_complex<d_t>(x);
     test_complex_schur_decompose_complex<v_t>(x);
     test_complex_schur_decompose_complex<fd_t>(x);