PYBIND11_SILENCE_MSVC_C4127 (more similar to the approach for C4100).

Author: rwgk

include/pybind11/cast.h

@@ -384,7 +384,11 @@ template <typename StringType, bool IsView = false> struct string_caster {
 
         const auto *buffer = reinterpret_cast<const CharT *>(PYBIND11_BYTES_AS_STRING(utfNbytes.ptr()));
         size_t length = (size_t) PYBIND11_BYTES_SIZE(utfNbytes.ptr()) / sizeof(CharT);
-        if (constexpr_bool(UTF_N > 8)) { buffer++; length--; } // Skip BOM for UTF-16/32
+        // Skip BOM for UTF-16/32
+        if (PYBIND11_SILENCE_MSVC_C4127(UTF_N > 8)) {
+            buffer++;
+            length--;
+        }
         value = StringType(buffer, length);
 
         // If we're loading a string_view we need to keep the encoded Python object alive:
@@ -499,7 +503,7 @@ template <typename CharT> struct type_caster<CharT, enable_if_t<is_std_char_type
         // out how long the first encoded character is in bytes to distinguish between these two
         // errors.  We also allow want to allow unicode characters U+0080 through U+00FF, as those
         // can fit into a single char value.
-        if (constexpr_bool(StringCaster::UTF_N == 8) && str_len > 1 && str_len <= 4) {
+        if (PYBIND11_SILENCE_MSVC_C4127(StringCaster::UTF_N == 8) && str_len > 1 && str_len <= 4) {
             auto v0 = static_cast<unsigned char>(value[0]);
             // low bits only: 0-127
             // 0b110xxxxx - start of 2-byte sequence
@@ -524,7 +528,7 @@ template <typename CharT> struct type_caster<CharT, enable_if_t<is_std_char_type
         // UTF-16 is much easier: we can only have a surrogate pair for values above U+FFFF, thus a
         // surrogate pair with total length 2 instantly indicates a range error (but not a "your
         // string was too long" error).
-        else if (constexpr_bool(StringCaster::UTF_N == 16) && str_len == 2) {
+        else if (PYBIND11_SILENCE_MSVC_C4127(StringCaster::UTF_N == 16) && str_len == 2) {
             one_char = static_cast<CharT>(value[0]);
             if (one_char >= 0xD800 && one_char < 0xE000)
                 throw value_error("Character code point not in range(0x10000)");
@@ -778,7 +782,7 @@ struct pyobject_caster {
         // For Python 2, without this implicit conversion, Python code would
         // need to be cluttered with six.ensure_text() or similar, only to be
         // un-cluttered later after Python 2 support is dropped.
-        if (constexpr_bool(std::is_same<T, str>::value) && isinstance<bytes>(src)) {
+        if (PYBIND11_SILENCE_MSVC_C4127(std::is_same<T, str>::value) && isinstance<bytes>(src)) {
             PyObject *str_from_bytes = PyUnicode_FromEncodedObject(src.ptr(), "utf-8", nullptr);
             if (!str_from_bytes) throw error_already_set();
             value = reinterpret_steal<type>(str_from_bytes);

include/pybind11/detail/common.h

@@ -926,8 +926,16 @@ inline constexpr void workaround_incorrect_msvc_c4100(Args &&...) {}
 #    define PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(...)
 #endif
 
-// Suppresses MSVC warning C4127: Conditional expression is constant
-constexpr inline bool constexpr_bool(bool cond) { return cond; }
+#if defined(_MSC_VER) // All versions (as of July 2021).
+
+// warning C4127: Conditional expression is constant
+constexpr inline bool silence_msvc_c4127(bool cond) { return cond; }
+
+#    define PYBIND11_SILENCE_MSVC_C4127(...) detail::silence_msvc_c4127(__VA_ARGS__)
+
+#else
+#    define PYBIND11_SILENCE_MSVC_C4127(...) __VA_ARGS__
+#endif
 
 PYBIND11_NAMESPACE_END(detail)
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)

include/pybind11/detail/init.h

@@ -96,7 +96,7 @@ template <typename Class>
 void construct(value_and_holder &v_h, Cpp<Class> *ptr, bool need_alias) {
     PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(need_alias);
     no_nullptr(ptr);
-    if (constexpr_bool(Class::has_alias) && need_alias && !is_alias<Class>(ptr)) {
+    if (PYBIND11_SILENCE_MSVC_C4127(Class::has_alias) && need_alias && !is_alias<Class>(ptr)) {
         // We're going to try to construct an alias by moving the cpp type.  Whether or not
         // that succeeds, we still need to destroy the original cpp pointer (either the
         // moved away leftover, if the alias construction works, or the value itself if we
@@ -136,7 +136,7 @@ void construct(value_and_holder &v_h, Holder<Class> holder, bool need_alias) {
     auto *ptr = holder_helper<Holder<Class>>::get(holder);
     no_nullptr(ptr);
     // If we need an alias, check that the held pointer is actually an alias instance
-    if (constexpr_bool(Class::has_alias) && need_alias && !is_alias<Class>(ptr))
+    if (PYBIND11_SILENCE_MSVC_C4127(Class::has_alias) && need_alias && !is_alias<Class>(ptr))
         throw type_error("pybind11::init(): construction failed: returned holder-wrapped instance "
                          "is not an alias instance");
 
@@ -153,7 +153,7 @@ void construct(value_and_holder &v_h, Cpp<Class> &&result, bool need_alias) {
     PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(need_alias);
     static_assert(std::is_move_constructible<Cpp<Class>>::value,
         "pybind11::init() return-by-value factory function requires a movable class");
-    if (constexpr_bool(Class::has_alias) && need_alias)
+    if (PYBIND11_SILENCE_MSVC_C4127(Class::has_alias) && need_alias)
         construct_alias_from_cpp<Class>(is_alias_constructible<Class>{}, v_h, std::move(result));
     else
         v_h.value_ptr() = new Cpp<Class>(std::move(result));

include/pybind11/pybind11.h

@@ -157,7 +157,7 @@ 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 (constexpr_bool(sizeof(capture) <= sizeof(rec->data))) {
+        if (PYBIND11_SILENCE_MSVC_C4127(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. */

include/pybind11/pytypes.h

@@ -1191,9 +1191,9 @@ 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 (constexpr_bool(sizeof(Unsigned) <= sizeof(unsigned long))
+    if (PYBIND11_SILENCE_MSVC_C4127(sizeof(Unsigned) <= sizeof(unsigned long))
 #if PY_VERSION_HEX < 0x03000000
-            || PyInt_Check(o)
+        || PyInt_Check(o)
 #endif
     ) {
         unsigned long v = PyLong_AsUnsignedLong(o);
@@ -1212,7 +1212,7 @@ class int_ : public object {
     template <typename T,
               detail::enable_if_t<std::is_integral<T>::value, int> = 0>
     int_(T value) {
-        if (detail::constexpr_bool(sizeof(T) <= sizeof(long))) {
+        if (PYBIND11_SILENCE_MSVC_C4127(sizeof(T) <= sizeof(long))) {
             if (std::is_signed<T>::value)
                 m_ptr = PyLong_FromLong((long) value);
             else