text: #include <stan/math/prim/mat.hpp>
  #include <gtest/gtest.h>
  #include <vector>
  
  TEST(MetaTraits, length_mvt) {
    using stan::length_mvt;
  
    double x1;
    EXPECT_THROW(length_mvt(x1), std::invalid_argument);
  
    int x2;
    EXPECT_THROW(length_mvt(x2), std::invalid_argument);
  }
  