Change variable name it to curr_overl to improve readability, add…

… `assert()` in the only place where `curr_overl` could become `nullptr`.

include/pybind11/pybind11.h

@@ -719,7 +719,7 @@ class cpp_function : public function {
         /* Iterator over the list of potentially admissible overloads */
         const function_record *overloads = reinterpret_cast<function_record *>(
                                   PyCapsule_GetPointer(self, get_function_record_capsule_name())),
-                              *it = overloads;
+                              *curr_overl = overloads;
         assert(overloads != nullptr);
 
         /* Need to know how many arguments + keyword arguments there are to pick the right
@@ -757,9 +757,9 @@ class cpp_function : public function {
             std::vector<function_call> second_pass;
 
             // However, if there are no overloads, we can just skip the no-convert pass entirely
-            const bool overloaded = it != nullptr && it->next != nullptr;
+            const bool overloaded = curr_overl != nullptr && curr_overl->next != nullptr;
 
-            for (; it != nullptr; it = it->next) {
+            for (; curr_overl != nullptr; curr_overl = curr_overl->next) {
 
                 /* For each overload:
                    1. Copy all positional arguments we were given, also checking to make sure that
@@ -780,7 +780,7 @@ class cpp_function : public function {
                    a result other than PYBIND11_TRY_NEXT_OVERLOAD.
                  */
 
-                const function_record &func = *it;
+                const function_record &func = *curr_overl;
                 size_t num_args = func.nargs; // Number of positional arguments that we need
                 if (func.has_args) {
                     --num_args; // (but don't count py::args
@@ -1018,10 +1018,11 @@ class cpp_function : public function {
                     }
 
                     if (result.ptr() != PYBIND11_TRY_NEXT_OVERLOAD) {
-                        // The error reporting logic below expects 'it' to be valid, as it would be
-                        // if we'd encountered this failure in the first-pass loop.
+                        // The error reporting logic below expects 'curr_overl' to be valid, as it
+                        // would be if we'd encountered this failure in the first-pass loop.
                         if (!result) {
-                            it = &call.func;
+                            curr_overl = &call.func;
+                            assert(curr_overl != nullptr);
                         }
                         break;
                     }
@@ -1168,7 +1169,7 @@ class cpp_function : public function {
         if (!result) {
             std::string msg = "Unable to convert function return value to a "
                               "Python type! The signature was\n\t";
-            msg += it->signature;
+            msg += curr_overl->signature;
             append_note_if_missing_header_is_suspected(msg);
             // Attach additional error info to the exception if supported
             if (PyErr_Occurred()) {