From 95e6fac0a5a1eee3aa23e4da0a93c6c25e32fb98 Mon Sep 17 00:00:00 2001
From: Thiago Macieira <thiago.macieira@intel.com>
Date: Sat, 20 May 2023 08:37:03 -0700
Subject: [PATCH] Short-live qIsConstantEvaluated()

This is not q20::is_constant_evaluated() because it does not replace
that for all compilers. Instead, it's our own version of it that may
return false even in constant contexts. However, for the majority of our
users, it will work even in C++17 mode.

Updated QStringView and QAnyStringView to use it, which are the only two
places in all of Qt that used std::is_constant_evaluated().

Change-Id: Ieab617d69f3b4b54ab30fffd175c50c517589226
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
---
 src/corelib/global/qttypetraits.h               | 17 +++++++++++++++++
 src/corelib/text/qanystringview.h               | 15 ++++-----------
 src/corelib/text/qstringview.h                  |  4 ++--
 .../text/qanystringview/tst_qanystringview.cpp  |  6 +++++-
 4 files changed, 28 insertions(+), 14 deletions(-)

diff --git a/src/corelib/global/qttypetraits.h b/src/corelib/global/qttypetraits.h
index 49f2728e9f..9129f536aa 100644
--- a/src/corelib/global/qttypetraits.h
+++ b/src/corelib/global/qttypetraits.h
@@ -17,6 +17,23 @@
 
 QT_BEGIN_NAMESPACE
 
+// like std::is_constant_evaluated
+#define QT_SUPPORTS_IS_CONSTANT_EVALUATED
+#ifdef __cpp_lib_is_constant_evaluated
+constexpr bool qIsConstantEvaluated() noexcept
+{
+    return std::is_constant_evaluated();
+}
+#elif __has_builtin(__builtin_is_constant_evaluated) || \
+    (defined(Q_CC_MSVC_ONLY) /* >= 1925, but we require 1927 in qglobal.h */)
+constexpr bool qIsConstantEvaluated() noexcept
+{
+    return __builtin_is_constant_evaluated();
+}
+#else
+#  undef QT_SUPPORTS_IS_CONSTANT_EVALUATED
+#endif
+
 // like std::to_underlying
 template <typename Enum>
 constexpr std::underlying_type_t<Enum> qToUnderlying(Enum e) noexcept
diff --git a/src/corelib/text/qanystringview.h b/src/corelib/text/qanystringview.h
index c2a4b9ad09..ac5cb54c78 100644
--- a/src/corelib/text/qanystringview.h
+++ b/src/corelib/text/qanystringview.h
@@ -101,12 +101,12 @@ private:
     static constexpr bool isAsciiOnlyCharsAtCompileTime(Char *str, qsizetype sz) noexcept
     {
         // do not perform check if not at compile time
-#if !defined(__cpp_lib_is_constant_evaluated)
+#if !defined(QT_SUPPORTS_IS_CONSTANT_EVALUATED)
         Q_UNUSED(str);
         Q_UNUSED(sz);
         return false;
 #else
-        if (!std::is_constant_evaluated())
+        if (!qIsConstantEvaluated())
             return false;
         if constexpr (sizeof(Char) != sizeof(char)) {
             Q_UNUSED(str);
@@ -137,8 +137,8 @@ private:
     template <typename Char>
     static constexpr qsizetype lengthHelperPointer(const Char *str) noexcept
     {
-#ifdef __cpp_lib_is_constant_evaluated
-        if (std::is_constant_evaluated())
+#ifdef QT_SUPPORTS_IS_CONSTANT_EVALUATED
+        if (qIsConstantEvaluated())
             return qsizetype(std::char_traits<Char>::length(str));
 #endif
         if constexpr (sizeof(Char) == sizeof(char16_t))
@@ -289,13 +289,6 @@ public:
     [[nodiscard]] Q_CORE_EXPORT static int compare(QAnyStringView lhs, QAnyStringView rhs, Qt::CaseSensitivity cs = Qt::CaseSensitive) noexcept;
     [[nodiscard]] Q_CORE_EXPORT static bool equal(QAnyStringView lhs, QAnyStringView rhs) noexcept;
 
-    static constexpr inline bool detects_US_ASCII_at_compile_time =
-#ifdef __cpp_lib_is_constant_evaluated
-            true
-#else
-            false
-#endif
-            ;
     //
     // STL compatibility API:
     //
diff --git a/src/corelib/text/qstringview.h b/src/corelib/text/qstringview.h
index 24a860c1a6..e4c0c4dd8d 100644
--- a/src/corelib/text/qstringview.h
+++ b/src/corelib/text/qstringview.h
@@ -105,8 +105,8 @@ private:
     template <typename Char>
     static constexpr qsizetype lengthHelperPointer(const Char *str) noexcept
     {
-#if defined(__cpp_lib_is_constant_evaluated)
-        if (std::is_constant_evaluated())
+#if defined(QT_SUPPORTS_IS_CONSTANT_EVALUATED)
+        if (qIsConstantEvaluated())
             return std::char_traits<Char>::length(str);
 #endif
         return QtPrivate::qustrlen(reinterpret_cast<const char16_t *>(str));
diff --git a/tests/auto/corelib/text/qanystringview/tst_qanystringview.cpp b/tests/auto/corelib/text/qanystringview/tst_qanystringview.cpp
index d5a62887e1..fa55bf5db2 100644
--- a/tests/auto/corelib/text/qanystringview/tst_qanystringview.cpp
+++ b/tests/auto/corelib/text/qanystringview/tst_qanystringview.cpp
@@ -449,7 +449,8 @@ void tst_QAnyStringView::basics() const
 
 void tst_QAnyStringView::asciiLiteralIsLatin1() const
 {
-    if constexpr (QAnyStringView::detects_US_ASCII_at_compile_time) {
+#ifdef QT_SUPPORTS_IS_CONSTANT_EVALUATED
+    if constexpr (true) {
         constexpr bool asciiCstringIsLatin1 = QAnyStringView("Hello, World").isLatin1();
         QVERIFY(asciiCstringIsLatin1);
         constexpr bool asciiUtf8stringIsLatin1 = QAnyStringView(u8"Hello, World").isLatin1();
@@ -466,6 +467,9 @@ void tst_QAnyStringView::asciiLiteralIsLatin1() const
                 !QAnyStringView::fromArray(u8"Tørrfisk").isLatin1();
         QVERIFY(utf8StringArrayIsNotLatin1);
     }
+#else
+    QSKIP("Compile-detection of US-ASCII strings not possible with this compiler");
+#endif
 }
 
 template <typename StringBuilder>