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

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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>
This commit is contained in:
Niels Lohmann
2026-07-09 07:12:01 +02:00
parent f3d48e4f2f
commit 8917c42bbc
6 changed files with 45 additions and 6 deletions
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.github/workflows/ubuntu.yml
+6 -4
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@@ -237,10 +237,12 @@ jobs:
strategy:
fail-fast: false
matrix:
# matrix to empirically pin down which nvcc/CUDA versions reproduce #3907
# (a c++20 parse error in iteration_proxy.hpp's enable_borrowed_range) before
# attempting a source-level fix; 11.8 predates CUDA's C++20 support entirely.
cuda: ['11.8.0', '12.0.1', '12.1.1', '12.2.2', '12.3.2', '12.4.1', '12.5.1', '12.6.3']
# 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