clang 13 C++20 consteval error #2455

englercj · 2021-08-15T15:51:17Z

I'm getting a compilation error with 8.0.1 using clang 13:

In file included from dependencies/fmtlib.fmt/fmt-8.0.1/src/format.cc:8:
dependencies/fmtlib.fmt/fmt-8.0.1/include/fmt/format-inl.h:92:54: error: call to consteval function 'fmt::basic_format_string<char>::basic_format_string<char [21], 0>' is not a constant expression
  if (written < count) FMT_THROW(system_error(errno, "cannot write to file"));
                                                     ^
dependencies/fmtlib.fmt/fmt-8.0.1/include/fmt/core.h:463:15: note: non-constexpr function 'strlen' cannot be used in a constant expression
      size_ = std::strlen(reinterpret_cast<const char*>(s));
              ^
dependencies/fmtlib.fmt/fmt-8.0.1/include/fmt/core.h:2835:51: note: in call to 'basic_string_view(&"cannot write to file"[0])'
  FMT_CONSTEVAL basic_format_string(const S& s) : str_(s) {
                                                  ^
dependencies/fmtlib.fmt/fmt-8.0.1/include/fmt/format-inl.h:92:54: note: in call to 'basic_format_string("cannot write to file")'
  if (written < count) FMT_THROW(system_error(errno, "cannot write to file"));
                                                     ^

Here is the full invocation:

clang++ -Idependencies/fmtlib.fmt/fmt-8.0.1/include -MMD -MP -m64 -Werror -ffast-math -O0 -g -mavx -Wall -Wextra -std=c++20 -fno-exceptions -fno-rtti -mcx16 -fvisibility=hidden -fvisibility-inlines-hidden -Wundef -o $(OBJDIR)/format.o -c dependencies/fmtlib.fmt/fmt-8.0.1/src/format.cc

And my full clang version if it helps:

$ clang --version
Debian clang version 13.0.0-++20210811062604+6789c4564a4b-1~exp1~20210811163414.34
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

The text was updated successfully, but these errors were encountered:

vitaut · 2021-08-15T16:06:36Z

clang 13 hasn't been released yet. Do you mean the current development version of clang? Also do you have a godbolt repro?

englercj · 2021-08-15T16:12:07Z

I'm using the Debian apt nightly packages from llvm: https://apt.llvm.org/. I've also tried with 12 and get the same error. I haven't been able to get a reproduction in godbolt yet.

englercj · 2021-08-15T16:32:02Z

Just downgraded and confirmed I'm seeing the same thing with clang 12:

$ make
==== Building fmt (debug_x64) ====
format.cc
clang++   -MMD -MP -DHE_PLATFORM_LINUX -DHE_PLATFORM_API_POSIX -D_DEBUG -DDEBUG -DHE_CFG_DEBUG -DHE_CFG_MODULE_NAME=\"fmt\" -DHE_CFG_MODULE_STATIC=1 -Idependencies/fmtlib.fmt/fmt-8.0.1/include -m64 -Werror -ffast-math -O0 -g -mavx -Wall -Wextra -std=c++20 -fno-exceptions -fno-rtti -mcx16 -fvisibility=hidden -fvisibility-inlines-hidden -Wundef  -o "obj/linux-debug_x64/fmt/format.o" -MF "obj/linux-debug_x64/fmt/format.d" -c "dependencies/fmtlib.fmt/fmt-8.0.1/src/format.cc"
In file included from dependencies/fmtlib.fmt/fmt-8.0.1/src/format.cc:8:
dependencies/fmtlib.fmt/fmt-8.0.1/include/fmt/format-inl.h:92:54: error: call to consteval function 'fmt::basic_format_string<char>::basic_format_string<char [21], 0>' is not a constant expression
  if (written < count) FMT_THROW(system_error(errno, "cannot write to file"));
                                                     ^
dependencies/fmtlib.fmt/fmt-8.0.1/include/fmt/core.h:463:15: note: non-constexpr function 'strlen' cannot be used in a constant expression
      size_ = std::strlen(reinterpret_cast<const char*>(s));
              ^
dependencies/fmtlib.fmt/fmt-8.0.1/include/fmt/core.h:2835:51: note: in call to 'basic_string_view(&"cannot write to file"[0])'
  FMT_CONSTEVAL basic_format_string(const S& s) : str_(s) {
                                                  ^
dependencies/fmtlib.fmt/fmt-8.0.1/include/fmt/format-inl.h:92:54: note: in call to 'basic_format_string("cannot write to file")'
  if (written < count) FMT_THROW(system_error(errno, "cannot write to file"));
                                                     ^
1 error generated.
make[1]: *** [fmt.make:180: obj/linux-debug_x64/fmt/format.o] Error 1
make: *** [Makefile:57: fmt] Error 2

$ clang++ --version
Debian clang version 12.0.1-++20210804103842+fed41342a82f-1~exp1~20210804084624.136
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

phprus · 2021-08-15T16:57:32Z

libstdc++ from Debian 9 does not support constexpr std::char_traits.
libstdc++ from Debian 10 does not support is_constant_evaluated.
https://distrowatch.com/table.php?distribution=debian and https://en.cppreference.com/w/cpp/compiler_support

-std=c++20 is not fully compatible with libstdc++ 6.3.0, 8.3.0.

Please check code with libc++.

englercj · 2021-08-15T20:04:40Z

Upgrading libc++ did in-fact fix the issue, great catch there. Thanks for your help.

ndevenish · 2021-09-09T11:22:30Z

I also ran into this, and solved by downgrading my set(CMAKE_CXX_STANDARD 20) to set(CMAKE_CXX_STANDARD 17) (wasn't otherwise using C++20 features).

timblechmann · 2021-09-22T14:40:00Z

i'm seeing a slightly related error: appleclang (from xcode 12.5 and xcode 13) both ICE when compiling in c++20 mode. the ICE goes away when disabling the FMT_CONSTEVAL selection.

the code enables the consteval codepath when clang v11.1 is detected, though

kotori2 · 2021-09-24T17:27:48Z

i'm seeing a slightly related error: appleclang (from xcode 12.5 and xcode 13) both ICE when compiling in c++20 mode. the ICE goes away when disabling the FMT_CONSTEVAL selection.

the code enables the consteval codepath when clang v11.1 is detected, though

@timblechmann
Similar issue here. It seems this bug was introduced somewhere between fmtlib 70103 and 80001.
I got the same error: call to consteval function 'fmt::basic_format_string<char, char *&>::basic_format_string<std::basic_string_view<char>, 0>' is not a constant expression error.
Currently using Apple clang version 13.0.0 (clang-1300.0.29.3) on macOS 12.0 beta.

mwinterb · 2021-09-24T17:40:51Z

i'm seeing a slightly related error: appleclang (from xcode 12.5 and xcode 13) both ICE when compiling in c++20 mode. the ICE goes away when disabling the FMT_CONSTEVAL selection.
the code enables the consteval codepath when clang v11.1 is detected, though

@timblechmann
Similar issue here. It seems this bug was introduced somewhere between fmtlib 70103 and 80001.
I got the same error: call to consteval function 'fmt::basic_format_string<char, char *&>::basic_format_string<std::basic_string_view<char>, 0>' is not a constant expression error.
Currently using Apple clang version 13.0.0 (clang-1300.0.29.3) on macOS 12.0 beta.

Your issue sounds more like #2438.

kotori2 · 2021-09-24T18:27:52Z

@mwinterb I just realized the error was followed by a compiler crash.
There is an example to reproduce:

#include <fmt/format.h>

template<typename ... Args>
static void output_logs() {
    fmt::format("");
}

int main() {
    output_logs();
}

[ 50%] Building CXX object CMakeFiles/untitled.dir/main.cpp.o
clang: error: unable to execute command: Segmentation fault: 11
clang: error: clang frontend command failed due to signal (use -v to see invocation)
Apple clang version 13.0.0 (clang-1300.0.29.3)
Target: x86_64-apple-darwin21.1.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
clang: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang: note: diagnostic msg: /var/folders/cj/z4mkrrn51rlfcqvwq7_9pmfw0000gn/T/main-b1386c.cpp
clang: note: diagnostic msg: /var/folders/cj/z4mkrrn51rlfcqvwq7_9pmfw0000gn/T/main-b1386c.sh
clang: note: diagnostic msg: Crash backtrace is located in
clang: note: diagnostic msg: /Users/kotori0/Library/Logs/DiagnosticReports/clang_<YYYY-MM-DD-HHMMSS>_<hostname>.crash
clang: note: diagnostic msg: (choose the .crash file that corresponds to your crash)
clang: note: diagnostic msg: 

********************
make[3]: *** [CMakeFiles/untitled.dir/main.cpp.o] Error 254
make[2]: *** [CMakeFiles/untitled.dir/all] Error 2
make[1]: *** [CMakeFiles/untitled.dir/rule] Error 2
make: *** [untitled] Error 2

vitaut · 2021-09-24T18:55:03Z

@kotori2, please open a new issue since it's unrelated to the current one.

timblechmann · 2021-09-25T03:49:32Z

@kotori2's issue reduces to:

struct basic_format_string {
  template <typename S> consteval basic_format_string(S);
};
template <typename...> using format_string = basic_format_string;
auto format(format_string<>) -> int;
template <typename...> void output_logs() { format(""); }
int main() { output_logs(); }

vitaut · 2021-09-25T13:42:49Z

consteval is now disabled in Apple clang because of https://bugs.llvm.org/show_bug.cgi?id=51938.

ndevenish · 2021-09-30T13:22:02Z

I'm also seeing this on linux, and now want to use some C++20 features. Is it worth adding a configuration check for (presumably) partially missing standard library support for this?

vitaut · 2021-09-30T13:37:10Z

@ndevenish, there have been several unrelated issue reported here. What error do you get?

ndevenish · 2021-09-30T13:55:44Z

Roughly equivalent to the original issue:

fmt/include/fmt/os.h:495:49: error: call to consteval function 'fmt::basic_format_string<char>::basic_format_string<char [21], 0>' is not a constant expression
    if (!locale_) FMT_THROW(system_error(errno, "cannot create locale"));
                                                ^
/home/user101/miniapp/dependencies/fmt/include/fmt/core.h:463:15: note: non-constexpr function 'strlen' cannot be used in a constant expression
      size_ = std::strlen(reinterpret_cast<const char*>(s));

Hmm, did some further digging, and maybe I jumped a bit too soon on the suggestion of feature checks. Although it works fine in compiler C++17 mode, and the clang-based compiler (dpcpp) i'm using is supposed to have up-to-date (if not full) C++20 support, it looks like the setup I am given has GCC 7.4's libstdc++ underneath, which would probably imply very little C++20 existing on the library side.

I suppose fixing this in features would require supporting C++20 the language changes without any C++20 The Standard Library changes, which probably isn't a reasonable.

vitaut · 2021-09-30T14:04:15Z

I think it's just a matter of refining the check here:

fmt/include/fmt/core.h

Lines 461 to 462 in d141cdb

    
           if (detail::const_check(std::is_same<Char, char>::value && 
        
                                   !detail::is_constant_evaluated()))

to account for unimplemented is_constant_evaluated in the standard library. A PR is welcome.

phprus · 2021-09-30T14:19:43Z

@vitaut, Why is this if statement needed?

std::char_traits<Char>::length(s) uses strlen if possible.

ndevenish · 2021-09-30T14:25:16Z

It seems to specifically be using std::strlen in cases where it detects that the standard library is not constant evaluated here. Is this a bug or I am misunderstanding the check?

fmt/include/fmt/core.h

Lines 352 to 358 in d141cdb

    
           constexpr FMT_INLINE auto is_constant_evaluated() FMT_NOEXCEPT -> bool { 
        
           #ifdef __cpp_lib_is_constant_evaluated 
        
             return std::is_constant_evaluated(); 
        
           #else 
        
             return false; 
        
           #endif 
        
           }

vitaut · 2021-09-30T14:26:12Z

Why is this if statement needed?

To get sensible code in debug mode: https://godbolt.org/z/56W3YG5xe.

vitaut · 2021-09-30T14:28:56Z

Is this a bug or I am misunderstanding the check?

It just doesn't play well with the fallback which is why it needs to be refined.

Also added commentary for why std::strlen is directly called in certain situations. Addresses fmtlib#2455 (comment).

vitaut · 2021-10-02T13:55:05Z

@ndevenish do you have a godbolt repro for the issue?

vitaut · 2021-10-09T13:19:41Z

This should be fixed now.

timblechmann · 2022-02-13T02:31:58Z

haven't been able to test, yet, but apparently fixed it in xcode13.3

Seems like the version of clang Github actions (it is using 13.x) doesn't fully support the C++20 features being used by googletest. From fmtlib/fmt#2455, the only fix was to downgrade to C++17, but we don't want to do that

englercj closed this as completed Aug 15, 2021

bigwhite37 mentioned this issue Sep 22, 2021

‘fmt’ is not a constant expression scylladb/seastar#950

Closed

vitaut reopened this Sep 30, 2021

mwinterb added a commit to mwinterb/fmt that referenced this issue Sep 30, 2021

Made basic_string_view's const Char* constructor constexpr in C++17.

3da6d3d

Also added commentary for why std::strlen is directly called in certain situations. Addresses fmtlib#2455 (comment).

mwinterb mentioned this issue Sep 30, 2021

Made basic_string_view's const Char* constructor constexpr in C++17. #2525

Closed

vitaut closed this as completed Oct 9, 2021

tt4g mentioned this issue Oct 17, 2021

"call to non-‘constexpr’ function" in bundled fmt gabime/spdlog#2142

Closed

timblechmann mentioned this issue Dec 23, 2021

C++20 crashes Apple-Clang 13.0.0 compiler on macos-11 (from Github Actions) gabime/spdlog#2219

Closed

Tectu mentioned this issue Jan 13, 2022

Support MacOS Tectu/malloy#46

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project triggers sigsegv on macos clang cpp-best-practices/gui_starter_template#180

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tt4g mentioned this issue Jul 10, 2022

"the value of 'm' is not usable in a constant expression" When Invoking Logging Function gabime/spdlog#2430

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This was referenced Aug 10, 2022

Workaround FreeBSD/clang/fmt bug dolphin-emu/dolphin#10973

Merged

consteval error with FreeBSD 13.1 and Clang13 #3032

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clang 13 C++20 consteval error #2455

clang 13 C++20 consteval error #2455

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clang 13 C++20 consteval error #2455

clang 13 C++20 consteval error #2455

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