Is the three-way comparison operator (C++20) supported in Qt 6.9 (clang and libc++) for `QString`, `QDate` and `QTime`?

pehg

I'm implementing a three-way comparison operator for a class. In such implementation, I use the <=> operator with some QString, QDate and QTime objects. According to this post https://www.qt.io/blog/c20-comparisons-in-qt, the operator was added to those classes in Qt 6.7. It's not in the documentation, though.

I have 3 setups described in the table below. The program compiles and runs perfectly in setups 2 and 3. However, it fails in setup 1.

Setups

	Setup 1	Setup 2	Setup 3
OS	freebsd 14	opensuse tumbleweed	tuxedo OS (ubuntu-based)
Compiler	Clang 18	GCC 15	GCC 15
C++ standard	C++23	C++23	C++23
Std lib	libc++	libstdc++	libstdc++
Qt	6.9	6.9	6.8
Type	CI job	CI job	local machine
Result	Compilation failure	Success	Success

Error

error: invalid operands to binary expression ('QDate' and 'QDate')
  142 |     if (const auto cmp = other.lastMessageDateTime.date() <=> lastMessageDateTime.date(); cmp != std::strong_ordering::equal) {
      |                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~

error: invalid operands to binary expression ('QTime' and 'QTime')
  145 |     if (const auto cmp = other.lastMessageDateTime.time() <=> lastMessageDateTime.time(); cmp != std::strong_ordering::equal) {
      |                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~

error: invalid operands to binary expression ('QString' and 'QString')
  149 |     if (const auto cmp = displayName() <=> other.displayName(); cmp != std::strong_ordering::equal) {
      |                          ~~~~~~~~~~~~~ ^   ~~~~~~~~~~~~~~~~~~~

error: invalid operands to binary expression ('const QString' and 'const QString')
  153 |     return accountJid <=> other.accountJid;
      |            ~~~~~~~~~~ ^   ~~~~~~~~~~~~~~~~

I tried compiling the project using clang on my local machine and it worked, but I had to use libstdc++. Apparently, I would need to compile Qt with libc++ to be able to use libc++. So, it's not the same configuration as the CI job.

So far the only solution I've found is to write an implementation that avoids using <=> with such Qt classes. I don't have a clue what could be going on in this case.

Kent-Dorfman

Does your Makefile explicitely use the clang option for c++20? Might it be defaulting to an older standard? Look at your generated Makefile for CXXOPTS or some such variant.

pehg

@Kent-Dorfman I don't think so, because in the same implementation I use the <=> operator with a member variable that is of type int. Additionally, there is no problem with the usage of std::strong_ordering and the project uses other C++20 features. The compiler complaints only about those Qt types using <=>.

SGaist

@pehg hi,

While taking a quick look at the header file mentioned in the article I stumbled upon this issue on the llvm project that might explain the situation.

I think you don't have __cpp_lib_three_way_comparison defined with clang 18.

pehg

@SGaist Hi,
I remember I saw that LLVM issue but I didn't know how to interpret it and I thought it was fixed. However, I just put the static asserts below in the code and compilation failed in the CI using clang and libc++, while it succeeded in the rest using GCC and libstdc++.

#include <compare>

// Check if the types support three-way comparison
static_assert(std::three_way_comparable<QString>, " ** QString - three-way comparison NOT supported");
static_assert(std::three_way_comparable<QDate>, " ** QDate - three-way comparison NOT supported");
static_assert(std::three_way_comparable<QTime>, " ** QTime - three-way comparison NOT supported");

// Check if the C++20 three-way comparison is available
static_assert(__cpp_lib_three_way_comparison >= 201907L, " ** Three-way comparison library NOT available");

I don't know if you refer to qcompare.h and qcomparehelpers.h. I just looked at them and I found usage of __cpp_lib_three_way_comparison . So, most likely my problem is due to the libc++ issue in the link you mentioned.