Throw an erorr if split is called with the same older and inner var name

src/Func.cpp

@@ -1010,6 +1010,12 @@ void Stage::split(const string &old, const string &outer, const string &inner, c
 
     definition.schedule().touched() = true;
 
+    user_assert(inner != outer) << "In schedule for " << name()
+                                << ", can't split " << old << " into "
+                                << outer << " and " << inner
+                                << " because the new Vars have the same name.\n"
+                                << dump_argument_list();
+
     // Check that the new names aren't already in the dims list.
     for (auto &dim : dims) {
         string new_names[2] = {inner, outer};

test/error/CMakeLists.txt

@@ -91,6 +91,7 @@ tests(GROUPS error
       specialize_fail.cpp
       split_inner_wrong_tail_strategy.cpp
       split_non_innermost_predicated.cpp
+      split_same_var_names.cpp
       thread_id_outside_block_id.cpp
       too_many_args.cpp
       tuple_arg_select_undef.cpp

test/error/split_same_var_names.cpp

@@ -0,0 +1,13 @@
+#include "Halide.h"
+
+using namespace Halide;
+
+int main(int argc, char **argv) {
+    Var x;
+    Func f;
+    f(x) = x;
+    f.split(x, x, x, 16, TailStrategy::RoundUp);
+
+    printf("Success!\n");
+    return 0;
+}