Use if constexpr

Author: cyyever

include/pybind11/detail/common.h

@@ -118,6 +118,12 @@
 #    endif
 #endif
 
+#if defined(__cpp_if_constexpr)
+#    define PYBIND11_IF_CONSTEXPR constexpr
+#else
+#    define PYBIND11_IF_CONSTEXPR
+#endif
+
 #if defined(PYBIND11_CPP20)
 #    define PYBIND11_CONSTINIT constinit
 #    define PYBIND11_DTOR_CONSTEXPR constexpr

include/pybind11/pybind11.h

@@ -222,7 +222,7 @@ class cpp_function : public function {
         auto *rec = unique_rec.get();
 
         /* Store the capture object directly in the function record if there is enough space */
-        if (sizeof(capture) <= sizeof(rec->data)) {
+        if PYBIND11_IF_CONSTEXPR (sizeof(capture) <= sizeof(rec->data)) {
             /* Without these pragmas, GCC warns that there might not be
                enough space to use the placement new operator. However, the
                'if' statement above ensures that this is the case. */
@@ -240,7 +240,7 @@ class cpp_function : public function {
 
             // UB without std::launder, but without breaking ABI and/or
             // a significant refactoring it's "impossible" to solve.
-            if (!std::is_trivially_destructible<capture>::value) {
+            if PYBIND11_IF_CONSTEXPR (!std::is_trivially_destructible<capture>::value) {
                 rec->free_data = [](function_record *r) {
                     auto data = PYBIND11_STD_LAUNDER((capture *) &r->data);
                     (void) data;
@@ -349,7 +349,7 @@ class cpp_function : public function {
         using FunctionType = Return (*)(Args...);
         constexpr bool is_function_ptr
             = std::is_convertible<Func, FunctionType>::value && sizeof(capture) == sizeof(void *);
-        if (is_function_ptr) {
+        if PYBIND11_IF_CONSTEXPR (is_function_ptr) {
             rec->is_stateless = true;
             rec->data[1]
                 = const_cast<void *>(reinterpret_cast<const void *>(&typeid(FunctionType)));
@@ -1595,7 +1595,7 @@ class class_ : public detail::generic_type {
 
         generic_type::initialize(record);
 
-        if (has_alias) {
+        if PYBIND11_IF_CONSTEXPR (has_alias) {
             auto &instances = record.module_local ? get_local_internals().registered_types_cpp
                                                   : get_internals().registered_types_cpp;
             instances[std::type_index(typeid(type_alias))]

include/pybind11/pytypes.h

@@ -1797,7 +1797,7 @@ PYBIND11_NAMESPACE_BEGIN(detail)
 // unsigned type: (A)-1 != (B)-1 when A and B are unsigned types of different sizes).
 template <typename Unsigned>
 Unsigned as_unsigned(PyObject *o) {
-    if (sizeof(Unsigned) <= sizeof(unsigned long)) {
+    if PYBIND11_IF_CONSTEXPR (sizeof(Unsigned) <= sizeof(unsigned long)) {
         unsigned long v = PyLong_AsUnsignedLong(o);
         return v == (unsigned long) -1 && PyErr_Occurred() ? (Unsigned) -1 : (Unsigned) v;
     }
@@ -1814,7 +1814,7 @@ class int_ : public object {
     template <typename T, detail::enable_if_t<std::is_integral<T>::value, int> = 0>
     // NOLINTNEXTLINE(google-explicit-constructor)
     int_(T value) {
-        if (sizeof(T) <= sizeof(long)) {
+        if PYBIND11_IF_CONSTEXPR (sizeof(T) <= sizeof(long)) {
             if (std::is_signed<T>::value) {
                 m_ptr = PyLong_FromLong((long) value);
             } else {