Replace uses of std::is_trivial with its underlying definitions

C++26 deprecated it[1]. Like the paper, we don't need the trivial
default constructibility for most uses. Even some of the
QTypeInfo::isComplex checks don't need it.

[1] https://wg21.link/p3247

Pick-to: 6.8 6.5
Fixes: QTBUG-136083
Change-Id: I0caafc5a15df732b9640fffd72fb1c598563a91b
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 97f3d41fa15a1dcaa17993436b9385ebd0fd274b)

src/corelib/global/qtypeinfo.h

@@ -20,6 +20,18 @@ class QDebug;
 
 namespace QtPrivate {
 
+// Helper for QTypeInfo<T>::isComplex, which used to be simply
+// !std::is_trivial_v but P3247 deprecated it for C++26. It used to be defined
+// (since C++11) by [class]/7 as: "A trivial class is a class that is trivially
+// copyable and has one or more default constructors, all of which are either
+// trivial or deleted and at least one of which is not deleted. [ Note: In
+// particular, a trivially copyable or trivial class does not have virtual
+// functions or virtual base classes. — end note ]".
+
+template <typename T>
+inline constexpr bool qIsComplex =
+        !std::is_trivially_default_constructible_v<T> || !std::is_trivially_copyable_v<T>;
+
 // A trivially copyable class must also have a trivial, non-deleted
 // destructor [class.prop/1.3], CWG1734. Some implementations don't
 // check for a trivial destructor, because of backwards compatibility
@@ -53,7 +65,7 @@ public:
     enum {
         isPointer [[deprecated("Use std::is_pointer instead")]] = std::is_pointer_v<T>,
         isIntegral [[deprecated("Use std::is_integral instead")]] = std::is_integral_v<T>,
-        isComplex = !std::is_trivial_v<T>,
+        isComplex = QtPrivate::qIsComplex<T>,
         isRelocatable = QtPrivate::qIsRelocatable<T>,
         isValueInitializationBitwiseZero = QtPrivate::qIsValueInitializationBitwiseZero<T>,
     };
@@ -162,7 +174,7 @@ class QTypeInfo<TYPE > \
 { \
 public: \
     enum { \
-        isComplex = (((FLAGS) & Q_PRIMITIVE_TYPE) == 0) && !std::is_trivial_v<TYPE>, \
+        isComplex = (((FLAGS) & Q_PRIMITIVE_TYPE) == 0) && QtPrivate::qIsComplex<TYPE>, \
         isRelocatable = !isComplex || ((FLAGS) & Q_RELOCATABLE_TYPE) || QtPrivate::qIsRelocatable<TYPE>, \
         isPointer [[deprecated("Use std::is_pointer instead")]] = std::is_pointer_v< TYPE >, \
         isIntegral [[deprecated("Use std::is_integral instead")]] = std::is_integral< TYPE >::value, \

src/corelib/io/qprocess_unix.cpp

@@ -180,7 +180,7 @@ struct ChildError
     int code;
     char function[_POSIX_PIPE_BUF - sizeof(code)];
 };
-static_assert(std::is_trivial_v<ChildError>);
+static_assert(std::is_trivially_copy_constructible_v<ChildError>);
 #ifdef PIPE_BUF
 static_assert(PIPE_BUF >= sizeof(ChildError)); // PIPE_BUF may be bigger
 #endif

src/corelib/kernel/qobject_p_p.h

@@ -64,7 +64,7 @@ struct QObjectPrivate::ConnectionOrSignalVector
         Connection *next;
     };
 };
-static_assert(std::is_trivial_v<QObjectPrivate::ConnectionOrSignalVector>);
+static_assert(std::is_trivially_copyable_v<QObjectPrivate::ConnectionOrSignalVector>);
 
 struct QObjectPrivate::Connection : public ConnectionOrSignalVector
 {
@@ -130,8 +130,8 @@ struct QObjectPrivate::SignalVector : public ConnectionOrSignalVector
     }
     int count() const { return static_cast<int>(allocated); }
 };
-static_assert(
-        std::is_trivial_v<QObjectPrivate::SignalVector>); // it doesn't need to be, but it helps
+// it doesn't need to be, but it helps
+static_assert(std::is_trivially_copyable_v<QObjectPrivate::SignalVector>);
 
 struct QObjectPrivate::ConnectionData
 {

src/corelib/serialization/qcborvalue_p.h

@@ -89,7 +89,8 @@ struct ByteData
     QStringView asStringView() const{ return QStringView(utf16(), len / 2); }
     QString asQStringRaw() const    { return QString::fromRawData(utf16(), len / 2); }
 };
-static_assert(std::is_trivial<ByteData>::value);
+static_assert(std::is_trivially_default_constructible<ByteData>::value);
+static_assert(std::is_trivially_copyable<ByteData>::value);
 static_assert(std::is_standard_layout<ByteData>::value);
 } // namespace QtCbor