diff --git a/stan/math/rev/core/arena_matrix.hpp b/stan/math/rev/core/arena_matrix.hpp
index 64467e19a35..ed5ee3102ab 100644
--- a/stan/math/rev/core/arena_matrix.hpp
+++ b/stan/math/rev/core/arena_matrix.hpp
@@ -96,7 +96,8 @@ class arena_matrix : public Eigen::Map<MatrixType> {
             auto other
                 = make_chainable_ptr(plain_type_t<MatrixType>(std::move(x)));
             // other has it's rows and cols swapped already if it needed that
-            return base_map_t(&(other->coeffRef(0)), other->rows(), other->cols());
+            return base_map_t(&(other->coeffRef(0)), other->rows(),
+                              other->cols());
           } else {
             base_map_t map(
                 ChainableStack::instance_->memalloc_.alloc_array<Scalar>(
@@ -156,29 +157,28 @@ class arena_matrix : public Eigen::Map<MatrixType> {
   template <typename T, require_not_arena_matrix_t<T>* = nullptr>
   arena_matrix& operator=(T&& a) {
     using T_t = std::decay_t<T>;
-    if (std::is_rvalue_reference<T&&>::value
-        && is_plain_type<T_t>::value) {
-        // Note: plain_type_t here does nothing since T_t is plain type
-        auto other
-            = make_chainable_ptr(plain_type_t<MatrixType>(std::move(a)));
-         new (this) Base(&(other->coeffRef(0)), other->rows(), other->cols());
-        return *this;       
+    if (std::is_rvalue_reference<T&&>::value && is_plain_type<T_t>::value) {
+      // Note: plain_type_t here does nothing since T_t is plain type
+      auto other = make_chainable_ptr(plain_type_t<MatrixType>(std::move(a)));
+      new (this) Base(&(other->coeffRef(0)), other->rows(), other->cols());
+      return *this;
     } else {
-        // do we need to transpose?
-        if ((RowsAtCompileTime == 1 && T_t::ColsAtCompileTime == 1)
-            || (ColsAtCompileTime == 1 && T_t::RowsAtCompileTime == 1)) {
-            // placement new changes what data map points to - there is no allocation
-            new (this) Base(
-                ChainableStack::instance_->memalloc_.alloc_array<Scalar>(a.size()),
-                a.cols(), a.rows());
-
-        } else {
-            new (this) Base(
-                ChainableStack::instance_->memalloc_.alloc_array<Scalar>(a.size()),
-                a.rows(), a.cols());
-        }
-        Base::operator=(a);
-        return *this;
+      // do we need to transpose?
+      if ((RowsAtCompileTime == 1 && T_t::ColsAtCompileTime == 1)
+          || (ColsAtCompileTime == 1 && T_t::RowsAtCompileTime == 1)) {
+        // placement new changes what data map points to - there is no
+        // allocation
+        new (this) Base(
+            ChainableStack::instance_->memalloc_.alloc_array<Scalar>(a.size()),
+            a.cols(), a.rows());
+
+      } else {
+        new (this) Base(
+            ChainableStack::instance_->memalloc_.alloc_array<Scalar>(a.size()),
+            a.rows(), a.cols());
+      }
+      Base::operator=(a);
+      return *this;
     }
   }
 };
diff --git a/test/unit/math/rev/fun/fill_test.cpp b/test/unit/math/rev/fun/fill_test.cpp
index fa328aa0a68..7dae163c0f7 100644
--- a/test/unit/math/rev/fun/fill_test.cpp
+++ b/test/unit/math/rev/fun/fill_test.cpp
@@ -60,7 +60,8 @@ TEST_F(AgradRev, fillVarMatDouble) {
   using stan::math::fill;
   using stan::math::sum;
   using stan::math::var_value;
-  Matrix<double, Dynamic, 1> y_val = Matrix<double, Dynamic, 1>::Constant(3, 2.0);
+  Matrix<double, Dynamic, 1> y_val
+      = Matrix<double, Dynamic, 1>::Constant(3, 2.0);
   var_value<Matrix<double, Dynamic, 1>> y(y_val);
   fill(y, 3.0);
   EXPECT_EQ(3, y.size());
@@ -79,7 +80,8 @@ TEST_F(AgradRev, fillVarMatVar) {
   using stan::math::sum;
   using stan::math::var;
   using stan::math::var_value;
-  Matrix<double, Dynamic, 1> y_val = Matrix<double, Dynamic, 1>::Constant(3, 2.0);
+  Matrix<double, Dynamic, 1> y_val
+      = Matrix<double, Dynamic, 1>::Constant(3, 2.0);
   var_value<Matrix<double, Dynamic, 1>> y(y_val);
   var z(3.0);
   fill(y, z);