Corrade with BUILD_TESTS=ON compilation error: call to non-‘constexpr’ function

[cmey]

Hi!

cmake -DBUILD_TESTS=ON ../
cmake --build .

fails with:

/home/cmey/Code/cpp/wonderland_repos/corrade/src/Corrade/Utility/Assert.h:242:6: error: call to non-‘constexpr’ function ‘Corrade::Utility::Test::{anonymous}::divide(int, int)::<lambda()>’
  239 |     static_cast<void>((condition) ? 0 : ([&]() {                            \
      |                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  240 |         Corrade::Utility::Error{Corrade::Utility::Error::defaultOutput()} << message; \
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  241 |         std::abort();                                                       \
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  242 |     }(), 0))
      |     ~^~

/home/cmey/Code/cpp/wonderland_repos/corrade/src/Corrade/Utility/Assert.h:239:42: note: ‘Corrade::Utility::Test::{anonymous}::divide(int, int)::<lambda()>’ is not usable as a ‘constexpr’ function because:
  239 |     static_cast<void>((condition) ? 0 : ([&]() {                            \

/home/cmey/Code/cpp/wonderland_repos/corrade/src/Corrade/Utility/Assert.h:239:42: note: lambdas are implicitly ‘constexpr’ only in C++17 and later
  239 |     static_cast<void>((condition) ? 0 : ([&]() {                            \

This is with g++ version: g++ (Ubuntu 11.2.0-19ubuntu1) 11.2.0
on Corrade branch master git # ba75f71, no local change.

Note: the build finishes correctly without BUILD_TESTS=ON.

[cmey]

And with clang++:

Ubuntu clang version 14.0.0-1ubuntu1
Target: x86_64-pc-linux-gnu
Thread model: posix

another constexpr-related error:

[ 63%] Building CXX object src/Corrade/Utility/Test/CMakeFiles/UtilityDebugAssertDisabledTest.dir/AssertDisabledTest.cpp.o
/home/cmey/Code/cpp/wonderland_repos/corrade/src/Corrade/Utility/Test/AssertDisabledTest.cpp:136:23: error: constexpr variable 'three' must be initialized by a constant expression
        constexpr int three = divide(15, 0);
                      ^       ~~~~~~~~~~~~~
/home/cmey/Code/cpp/wonderland_repos/corrade/src/Corrade/Utility/Test/AssertDisabledTest.cpp:123:12: note: non-literal type '(lambda at /home/cmey/Code/cpp/wonderland_repos/corrade/src/Corrade/Utility/Test/AssertDisabledTest.cpp:123:12)' cannot be used in a constant expression
    return TESTED_CONSTEXPR_ASSERT(b, "b can't be zero"), a/(b + 5);

[mosra]

Heh... unpossible. This has worked for ages on all compilers back to Clang 3.8, GCC 4.8 and MSVC 2015.

Both the GCC and the Clang error is about the same thing, a lambda that gets called if an expression fails. Which is a valid technique explained in this post from 2017. I don't know where the GCC error originates from, but I suspect it would be from AssertDisabledTest.cpp as well, since that's where this macro is mainly tested?

Like with #135, I have a suspicion there's something seriously broken with the toolchain you use 😅

[cmey]

The compilation error is reproducible in a clean Docker environment (docker run -it ubuntu:22.04) using clang-11, clang-12, clang-13, clang-14, and gcc-11 (haven't tried more) and also reproducible in archlinux docker image (docker run -it archlinux) using gcc:

[root@0e7cc222fbdc build]# cmake -DBUILD_TESTS=ON ../
-- The CXX compiler identification is GNU 12.1.0

[root@0e7cc222fbdc build]# cmake --build . --parallel
In file included from /tmp/corrade/src/Corrade/Containers/Pointer.h:37,
                 from /tmp/corrade/src/Corrade/TestSuite/Tester.h:35,
                 from /tmp/corrade/src/Corrade/Utility/Test/AssertDisabledTest.cpp:35:
/tmp/corrade/src/Corrade/Utility/Test/AssertDisabledTest.cpp: In member function ‘void Corrade::Utility::Test::{anonymous}::AssertDisabledTest::constexprTest()’:
/tmp/corrade/src/Corrade/Utility/Test/AssertDisabledTest.cpp:136:37:   in ‘constexpr’ expansion of ‘Corrade::Utility::Test::{anonymous}::divide(15, 0)’
/tmp/corrade/src/Corrade/Utility/Assert.h:242:6: error: call to non-‘constexpr’ function ‘Corrade::Utility::Test::{anonymous}::divide(int, int)::<lambda()>’
  239 |     static_cast<void>((condition) ? 0 : ([&]() {                            \
      |                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  240 |         Corrade::Utility::Error{Corrade::Utility::Error::defaultOutput()} << message; \
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  241 |         std::abort();                                                       \
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  242 |     }(), 0))
      |     ~^~
/tmp/corrade/src/Corrade/Utility/DebugAssert.h:86:5: note: in expansion of macro ‘CORRADE_CONSTEXPR_ASSERT’
   86 |     CORRADE_CONSTEXPR_ASSERT(condition, message)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~
/tmp/corrade/src/Corrade/Utility/Test/AssertDisabledTest.cpp:41:33: note: in expansion of macro ‘CORRADE_CONSTEXPR_DEBUG_ASSERT’
   41 | #define TESTED_CONSTEXPR_ASSERT CORRADE_CONSTEXPR_DEBUG_ASSERT
      |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/corrade/src/Corrade/Utility/Test/AssertDisabledTest.cpp:123:12: note: in expansion of macro ‘TESTED_CONSTEXPR_ASSERT’
  123 |     return TESTED_CONSTEXPR_ASSERT(b, "b can't be zero"), a/(b + 5);
      |            ^~~~~~~~~~~~~~~~~~~~~~~
/tmp/corrade/src/Corrade/Utility/Assert.h:239:42: note: ‘Corrade::Utility::Test::{anonymous}::divide(int, int)::<lambda()>’ is not usable as a ‘constexpr’ function because:
  239 |     static_cast<void>((condition) ? 0 : ([&]() {                            \
      |                                          ^
/tmp/corrade/src/Corrade/Utility/DebugAssert.h:86:5: note: in expansion of macro ‘CORRADE_CONSTEXPR_ASSERT’
   86 |     CORRADE_CONSTEXPR_ASSERT(condition, message)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~
/tmp/corrade/src/Corrade/Utility/Test/AssertDisabledTest.cpp:41:33: note: in expansion of macro ‘CORRADE_CONSTEXPR_DEBUG_ASSERT’
   41 | #define TESTED_CONSTEXPR_ASSERT CORRADE_CONSTEXPR_DEBUG_ASSERT
      |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/corrade/src/Corrade/Utility/Test/AssertDisabledTest.cpp:123:12: note: in expansion of macro ‘TESTED_CONSTEXPR_ASSERT’
  123 |     return TESTED_CONSTEXPR_ASSERT(b, "b can't be zero"), a/(b + 5);
      |            ^~~~~~~~~~~~~~~~~~~~~~~
/tmp/corrade/src/Corrade/Utility/Assert.h:239:42: note: lambdas are implicitly ‘constexpr’ only in C++17 and later
  239 |     static_cast<void>((condition) ? 0 : ([&]() {                            \
      |                                          ^
/tmp/corrade/src/Corrade/Utility/DebugAssert.h:86:5: note: in expansion of macro ‘CORRADE_CONSTEXPR_ASSERT’
   86 |     CORRADE_CONSTEXPR_ASSERT(condition, message)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~
/tmp/corrade/src/Corrade/Utility/Test/AssertDisabledTest.cpp:41:33: note: in expansion of macro ‘CORRADE_CONSTEXPR_DEBUG_ASSERT’
   41 | #define TESTED_CONSTEXPR_ASSERT CORRADE_CONSTEXPR_DEBUG_ASSERT
      |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/corrade/src/Corrade/Utility/Test/AssertDisabledTest.cpp:123:12: note: in expansion of macro ‘TESTED_CONSTEXPR_ASSERT’
  123 |     return TESTED_CONSTEXPR_ASSERT(b, "b can't be zero"), a/(b + 5);
      |            ^~~~~~~~~~~~~~~~~~~~~~~

[mosra]

Wait, I'm stupid. Sorry, realized the actual reason just now.

Yes, it fails even for me. The key was that CMAKE_BUILD_TYPE is unset by default, and ... tests for the CORRADE_DEBUG_ASSERT() macro don't handle that case properly, expecting either NDEBUG or CORRADE_IS_DEBUG_BUILD being set, and there's neither if CMAKE_BUILD_TYPE is empty. The AssertDisabledTest then passes a failing statement to the assert macro to test it's indeed not doing anything, which in turn means that a constexpr variable is not initialized by a constexpr statement (as that's how the assert failure manifests at compile time).

TL;DR: if you build with -DCMAKE_BUILD_TYPE=Release or Debug, it'll go through. Working on a proper fix now, apologies for assuming it's your toolchain being cursed and not my code being stupid.

[mosra]

Should be fixed with 1fea951, sorry again :)