[CMAKE] Upgrade TVM build baseline to C++20#19734
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Move TVM's core C++ and CUDA dialects to C++20 and update first-party helper build paths that compile TVM-generated C++ code. This keeps the codebase ready for C++20 native dependencies while leaving vendored third-party projects to manage their own minimum standards.
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Limit the C++20 baseline update to TVM's core CMake configuration and docs. Leave app, JVM, web, and Python helper compile flags at their existing C++17 settings so those downstream build surfaces can migrate separately.
Replace deprecated C++ traits and make captures explicit so the C++20 baseline builds cleanly under -Werror.
tqchen
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Jun 15, 2026
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## Summary This PR upgrades TVM's core CMake build baseline from C++17 to C++20. It updates the top-level CMake C++/CUDA standards and the LLVM compile probe fallback flags. Downstream app, JVM, web, Python helper, and vendored third-party build surfaces are intentionally left unchanged so they can migrate separately. ## Verification - Configured a minimal local build with GCC 13.3 and NVCC 13.2: `cmake -S . -B build/cxx20-check -GNinja -DBUILD_TESTING=OFF -DUSE_LLVM=OFF -DUSE_CUDA=OFF -DTVM_BUILD_PYTHON_MODULE=OFF` (cherry picked from commit c9a3c2f)
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Revert the CMake C++/CUDA standard from C++20 back to C++17, undoing the standard-setting portion of apache#19734 and restoring the v0.25.0.rc0 build baseline. The C++20 baseline broke the manylinux Linux wheels: under C++20, libstdc++'s constexpr std::string paths reference symbols newer than the manylinux_2_28 runtime baseline (e.g. _M_replace_cold @ GLIBCXX_3.4.30, GCC 12.1+), so the wheel fails to import with "undefined symbol". The compiler (GCC 14.2.1) is unchanged from rc0 -- only the standard differs -- so C++17 restores the working symbol set. Only the standard-setting surfaces are reverted (CMakeLists.txt, the LLVM compile probe, docs); the C++17-compatible source modernizations from apache#19734 are intentionally kept to avoid reintroducing deprecated constructs.
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Revert four explicit-this lambda captures ([=, this, ...] -> [=, ...]) in run_codegen.cc, inject_software_pipeline.cc, and cudnn_json_runtime.cc. These were introduced by the C++20 baseline upgrade (apache#19734); after reverting the baseline to C++17, MSVC rejects capturing 'this' explicitly under a '=' capture-default (error C3791) -- that form is C++20-only. GCC and Clang accept it as an extension in C++17, which is why only the Windows wheel build failed. Dropping the explicit 'this' ('=' captures it implicitly in C++17) restores the MSVC build with no behavior change.
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Summary
This PR upgrades TVM's core CMake build baseline from C++17 to C++20.
It updates the top-level CMake C++/CUDA standards and the LLVM compile probe fallback flags. Downstream app, JVM, web, Python helper, and vendored third-party build surfaces are intentionally left unchanged so they can migrate separately.
Verification
cmake -S . -B build/cxx20-check -GNinja -DBUILD_TESTING=OFF -DUSE_LLVM=OFF -DUSE_CUDA=OFF -DTVM_BUILD_PYTHON_MODULE=OFF