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Fix nvcc CUDA 12.0/12.1 C++20 ranges parse error (#3907) (#5248)

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* Test ci_cuda_example against a CUDA version matrix at C++20 (#3907)

The ci_cuda_example job compiled against the json-ci image's CUDA
11.0 toolkit at cuda_std_11, which cannot exercise #3907 (a c++20
parse error in iteration_proxy.hpp's enable_borrowed_range reported
under nvcc). Switch the job to pull official nvidia/cuda devel images
directly and matrix across CUDA 11.8-12.6 at cuda_std_20 so CI can
empirically confirm which versions are actually affected before any
source-level fix is attempted.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix nvcc CUDA 12.0/12.1 C++20 ranges parse error (#3907)

The diagnostic matrix in this PR confirmed the affected range exactly:
nvcc 12.0.1 and 12.1.1 both fail with "expected initializer before
'<' token" on iteration_proxy.hpp's enable_borrowed_range variable
template specialization at -std=c++20; 12.2.2 and newer already build
cleanly. Guard JSON_HAS_RANGES off for that narrow nvcc version range,
matching the existing GCC-11/libstdc++ carve-outs in the same ifdef
chain, and regenerate single_include accordingly.

Broaden the CUDA smoke test to also exercise comparisons
(operator==/operator<=>, gated independently by
JSON_HAS_THREE_WAY_COMPARISON) and range-based iteration, not just
dump()/erase(), so the fix's actual scope is evidenced by CI rather
than assumed from the single reported symptom.

Have tests/cuda_example/CMakeLists.txt pick the newest C++ standard
the detected nvcc version actually supports (20/17/11) instead of
hard-requiring C++20, so older toolkits build at a lower standard
instead of failing CMake configure outright. This is test-project-local
only; the JSON_HAS_RANGES guard is what protects real client code,
since a header can't control what -std= flag it's compiled with.

Right-size the CI matrix from the 8-version diagnostic sweep down to
11.8.0 (C++17 fallback path) / 12.1.1 (permanent #3907 regression
guard) / 12.6.3 (recent coverage), and update the compiler-version
table in the quality assurance docs to match.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix ci_cuda_example CUDA 11.8 build after C++17 fallback (#3907)

The 11.8.0 leg's graceful C++17 fallback (added in the previous commit)
worked correctly, but the broadened smoke test used the <=> operator
unconditionally, which isn't valid syntax pre-C++20 — nvcc rejected it
with "expected an expression" once the CMake logic picked cuda_std_17
for the older toolkit. Gate those two lines behind
JSON_HAS_THREE_WAY_COMPARISON like the library itself does internally.

Sanity-compiled the file as plain C++ at both -std=c++17 (skips the
guarded block) and -std=c++20 (includes it) locally; the actual nvcc
build is verified via CI on PR #5248.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-07-09 19:02:36 +02:00
committed by GitHub
parent 521a084827
commit f8e99e856c
7 changed files with 53 additions and 4 deletions
Download Patch File Download Diff File Expand all files Collapse all files
.github/workflows/ubuntu.yml
+12 -1
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@@ -234,11 +234,22 @@ jobs:
ci_cuda_example:
runs-on: ubuntu-latest
container: ghcr.io/nlohmann/json-ci:v2.4.0
strategy:
fail-fast: false
matrix:
# 11.8.0: newest pre-C++20 CUDA release, exercises the C++17 fallback
# path (tests/cuda_example/CMakeLists.txt picks the standard per nvcc
# version); 12.1.1: permanent regression guard for #3907 (nvcc 12.0/12.1
# choke on enable_borrowed_range at C++20, fixed in 12.2); 12.6.3: recent
# CUDA/C++20 coverage.
cuda: ['11.8.0', '12.1.1', '12.6.3']
container: nvidia/cuda:${{ matrix.cuda }}-devel-ubuntu22.04
steps:
- uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
with:
persist-credentials: false
- name: Get latest CMake and ninja
uses: lukka/get-cmake@f5b8fbb4d77cec1acc5a5f9f0df4beffaf5d98d9 # v4.3.4
- name: Run CMake
run: cmake -S . -B build -DJSON_CI=On
- name: Build
cmake/ci.cmake
-1
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@@ -669,7 +669,6 @@ add_custom_target(ci_test_compiler_default
add_custom_target(ci_cuda_example
COMMAND ${CMAKE_COMMAND}
-DCMAKE_BUILD_TYPE=Debug -GNinja
-DCMAKE_CUDA_HOST_COMPILER=g++-8
-S${PROJECT_SOURCE_DIR}/tests/cuda_example -B${PROJECT_BINARY_DIR}/build_cuda_example
COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/build_cuda_example
)
docs/mkdocs/docs/community/quality_assurance.md
+3 -1
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@@ -66,7 +66,9 @@ Note: Some modern features (like C++20 ranges or filesystem support) may be disa
| Clang 20.1.1 | x86_64 | Ubuntu 22.04.1 LTS | GitHub |
| Clang 20.1.8 with GNU-like command-line | x86_64 | Windows Server 2022 (Build 20348) | GitHub |
| Clang 21.1.8 | x86_64 | Ubuntu 22.04.1 LTS | GitHub |
| CUDA 11.0.221 (nvcc) | x86_64 | Ubuntu 20.04 LTS | GitHub |
| CUDA 11.8.0 (nvcc) | x86_64 | Ubuntu 22.04 LTS | GitHub |
| CUDA 12.1.1 (nvcc) | x86_64 | Ubuntu 22.04 LTS | GitHub |
| CUDA 12.6.3 (nvcc) | x86_64 | Ubuntu 22.04 LTS | GitHub |
| Emscripten 4.0.6 | x86_64 | Ubuntu 22.04.1 LTS | GitHub |
| GNU 4.8.5 | x86_64 | Ubuntu 22.04.1 LTS | GitHub |
| GNU 4.9.3 | x86_64 | Ubuntu 22.04.1 LTS | GitHub |
include/nlohmann/detail/macro_scope.hpp
+5
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@@ -146,6 +146,11 @@
#define JSON_HAS_RANGES 0
#elif defined(_LIBCPP_VERSION) && _LIBCPP_VERSION < 160000
#define JSON_HAS_RANGES 0
// nvcc CUDA 12.0/12.1 chokes on the enable_borrowed_range variable-template
// syntax when compiling as CUDA source; fixed in CUDA 12.2 (issue #3907)
#elif defined(__CUDACC__) && defined(__CUDACC_VER_MAJOR__) && __CUDACC_VER_MAJOR__ == 12 \
&& defined(__CUDACC_VER_MINOR__) && (__CUDACC_VER_MINOR__ == 0 || __CUDACC_VER_MINOR__ == 1)
#define JSON_HAS_RANGES 0
#elif defined(__cpp_lib_ranges)
#define JSON_HAS_RANGES 1
#else
single_include/nlohmann/json.hpp
+5
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@@ -2520,6 +2520,11 @@ JSON_HEDLEY_DIAGNOSTIC_POP
#define JSON_HAS_RANGES 0
#elif defined(_LIBCPP_VERSION) && _LIBCPP_VERSION < 160000
#define JSON_HAS_RANGES 0
// nvcc CUDA 12.0/12.1 chokes on the enable_borrowed_range variable-template
// syntax when compiling as CUDA source; fixed in CUDA 12.2 (issue #3907)
#elif defined(__CUDACC__) && defined(__CUDACC_VER_MAJOR__) && __CUDACC_VER_MAJOR__ == 12 \
&& defined(__CUDACC_VER_MINOR__) && (__CUDACC_VER_MINOR__ == 0 || __CUDACC_VER_MINOR__ == 1)
#define JSON_HAS_RANGES 0
#elif defined(__cpp_lib_ranges)
#define JSON_HAS_RANGES 1
#else
tests/cuda_example/CMakeLists.txt
+12 -1
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@@ -3,7 +3,18 @@ project(json_cuda LANGUAGES CUDA)
add_executable(json_cuda json_cuda.cu)
target_include_directories(json_cuda PRIVATE ../../include)
target_compile_features(json_cuda PUBLIC cuda_std_11)
# nvcc added C++20 support in CUDA 12.0 and C++17 in CUDA 11.0; pick the
# newest standard the detected compiler actually supports (see #3907)
# instead of hard-requiring one standard for every CUDA version.
if(CMAKE_CUDA_COMPILER_VERSION VERSION_GREATER_EQUAL 12.0)
set(json_cuda_std 20)
elseif(CMAKE_CUDA_COMPILER_VERSION VERSION_GREATER_EQUAL 11.0)
set(json_cuda_std 17)
else()
set(json_cuda_std 11)
endif()
target_compile_features(json_cuda PUBLIC cuda_std_${json_cuda_std})
set_target_properties(json_cuda PROPERTIES
CUDA_EXTENSIONS OFF
CUDA_STANDARD_REQUIRED ON
tests/cuda_example/json_cuda.cu
+16
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@@ -16,4 +16,20 @@ int main()
// regression for #3013 (ordered_json::reset() compile error with nvcc)
nlohmann::ordered_json metadata;
metadata.erase("key");
// exercise comparisons (operator==/operator<=>, gated by
// JSON_HAS_THREE_WAY_COMPARISON, independent of JSON_HAS_RANGES) and
// range-based iteration (exercises iteration_proxy/ranges machinery
// beyond just the enable_borrowed_range specialization) — see #3907
nlohmann::json a = {1, 2, 3};
nlohmann::json b = {1, 2, 3};
static_cast<void>(a == b);
#if JSON_HAS_THREE_WAY_COMPARISON
static_cast<void>(a <=> b); // *NOPAD*
static_cast<void>(a <=> 1); // *NOPAD*
#endif
for (const auto& element : a)
{
static_cast<void>(element);
}
}