Breaking: Use C++ 20

Summary:
X-link: facebook/react-native#39437

Have been running into places where C++ 20 makes life easier for use like `std::bit_cast` (that one is easy to polyfill), in-class member initializer support for bitfields, designated initializers, defaulted comparison operator, concepts instead of SFINAE, and probably more.

Our other infra is in the process of making this jump, or already has. This tests it out everywhere, across the various reference builds, to see if we have any issues.

This is a bit more aggressive than I had previously, but n - 1 is going to be a better long term place than n - 2. I think it is likely everything will be buildable for a while under Clang 10 (released 3.5 years ago), GCC 10 (releaseed 3.5 years ago), and VS 16.11 (released 2 years ago).

Differential Revision: D49261607

Package.swift

@@ -28,5 +28,5 @@ let package = Package(
             ]
         )
     ],
-    cxxLanguageStandard: CXXLanguageStandard(rawValue: "c++17")
+    cxxLanguageStandard: CXXLanguageStandard(rawValue: "c++20")
 )

README.md

@@ -4,7 +4,7 @@ Yoga is an embeddable and performant flexbox layout engine with bindings for mul
 
 
 ## Building
-Yoga's main implementation targets C++ 17 with accompanying build logic in CMake. A wrapper is provided to build the main library and run unit tests.
+Yoga's main implementation targets C++ 20 with accompanying build logic in CMake. A wrapper is provided to build the main library and run unit tests.
 
 ```sh
 ./unit_tests <Debug|Release>

Yoga.podspec

@@ -33,7 +33,7 @@ Pod::Spec.new do |spec|
       '-Werror',
       '-Wextra',
       '-Wconversion',
-      '-std=c++17',
+      '-std=c++20',
       '-fPIC'
   ]
 

cmake/project-defaults.cmake

@@ -3,7 +3,7 @@
 # This source code is licensed under the MIT license found in the
 # LICENSE file in the root directory of this source tree.
 
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_VISIBILITY_PRESET hidden)
 set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 

java/jni/YGJNIVanilla.cpp

@@ -5,19 +5,20 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-#include "YGJNIVanilla.h"
+#include <bit>
 #include <cstring>
 #include <iostream>
 #include <memory>
+
+#include <jni.h>
+#include <yoga/Yoga-internal.h>
+
 #include "LayoutContext.h"
 #include "YGJNI.h"
+#include "YGJNIVanilla.h"
 #include "YGJTypesVanilla.h"
 #include "YogaJniException.h"
 #include "common.h"
-#include "jni.h"
-
-#include <yoga/Yoga-internal.h>
-#include <yoga/bits/BitCast.h>
 
 using namespace facebook;
 using namespace facebook::yoga;
@@ -645,8 +646,8 @@ static YGSize YGJNIMeasureFunc(
     uint32_t wBits = 0xFFFFFFFF & (measureResult >> 32);
     uint32_t hBits = 0xFFFFFFFF & measureResult;
 
-    const float measuredWidth = yoga::bit_cast<float>(wBits);
-    const float measuredHeight = yoga::bit_cast<float>(hBits);
+    const float measuredWidth = std::bit_cast<float>(wBits);
+    const float measuredHeight = std::bit_cast<float>(hBits);
 
     return YGSize{measuredWidth, measuredHeight};
   } else {

javascript/CMakeLists.txt

@@ -14,7 +14,7 @@ file(GLOB SOURCES CONFIGURE_DEPENDS
 
 include_directories(..)
 
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 20)
 
 add_compile_definitions(
     EMSCRIPTEN_HAS_UNBOUND_TYPE_NAMES=0)

yoga/style/CompactValue.h

@@ -7,15 +7,14 @@
 
 #pragma once
 
+#include <bit>
 #include <cmath>
 #include <cstdint>
 #include <limits>
 
 #include <yoga/YGMacros.h>
 #include <yoga/YGValue.h>
 
-#include <yoga/bits/BitCast.h>
-
 static_assert(
     std::numeric_limits<float>::is_iec559,
     "facebook::yoga::detail::CompactValue only works with IEEE754 floats");
@@ -64,7 +63,7 @@ class YG_EXPORT CompactValue {
     }
 
     uint32_t unitBit = Unit == YGUnitPercent ? PERCENT_BIT : 0;
-    auto data = yoga::bit_cast<uint32_t>(value);
+    auto data = std::bit_cast<uint32_t>(value);
     data -= BIAS;
     data |= unitBit;
     return {data};
@@ -117,7 +116,7 @@ class YG_EXPORT CompactValue {
         return YGValue{0.0f, YGUnitPercent};
     }
 
-    if (std::isnan(yoga::bit_cast<float>(repr_))) {
+    if (std::isnan(std::bit_cast<float>(repr_))) {
       return YGValueUndefined;
     }
 
@@ -126,14 +125,14 @@ class YG_EXPORT CompactValue {
     data += BIAS;
 
     return YGValue{
-        yoga::bit_cast<float>(data),
+        std::bit_cast<float>(data),
         repr_ & 0x40000000 ? YGUnitPercent : YGUnitPoint};
   }
 
   bool isUndefined() const noexcept {
     return (
         repr_ != AUTO_BITS && repr_ != ZERO_BITS_POINT &&
-        repr_ != ZERO_BITS_PERCENT && std::isnan(yoga::bit_cast<float>(repr_)));
+        repr_ != ZERO_BITS_PERCENT && std::isnan(std::bit_cast<float>(repr_)));
   }
 
   bool isAuto() const noexcept {