build: Update C++ standard from c++17 to c++20

[mc-nv]

What does the PR do?

• Upstream PyTorch raised ATen's minimum required C++ standard from 17 to 20 ([12/12] Enforce C++20 minimum in header guards (#178150) pytorch/pytorch#178150), breaking the pytorch_backend build.
• Bumps this repo's C++ standard default from 17 to 20 for consistency across the Triton stack.

Checklist

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Commit Type:

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• build
• ci
• docs
• feat
• fix
• perf
• refactor
• revert
• style
• test

Related PRs:

• build: Update C++ standard from c++17 to c++20 backend#129
• build: Update C++ standard from c++17 to c++20 checksum_repository_agent#16
• build: Update C++ standard from c++17 to c++20 identity_backend#38
• build: Update C++ standard from c++17 to c++20 local_cache#18
• build: Update C++ standard from c++17 to c++20 openvino_backend#123
• build: Update C++ standard from c++17 to c++20 redis_cache#26
• build: Update C++ standard from c++17 to c++20 repeat_backend#21
• build: Update C++ standard from c++17 to c++20 square_backend#24
• build: Update C++ standard from c++17 to c++20 tensorrt_backend#128
• build: Update C++ standard from c++17 to c++20 third_party#82
• build: Update C++ standard from c++17 to c++20 common#166
• build: Update C++ standard from c++17 to c++20 core#525
• build: Update C++ standard from c++17 to c++20 client#922
• build: Update C++ standard from c++17 to c++20 onnxruntime_backend#359
• build: Update C++ standard from c++17 to c++20 server#8962
• build: Update C++ standard from c++17 to c++20 python_backend#455
• build: Update C++ standard from c++17 to c++20 fil_backend#461
• build: Update C++ standard from c++17 to c++20 dali_backend#311

Where should the reviewer start?

• CMakeLists.txt

Test plan:

• CI Pipeline ID: 67062220

Caveats:

This is the confirmed root cause repo — ATen.h now hard-errors below C++20 (see TRI-1877 description). Directly fixes the failing py3-base/*-rhel/-sbsa CI jobs.

Background

Part of a coordinated C++17->C++20 bump across ~18 Triton component repos (TRI-1877), triggered by upstream PyTorch's ATen.h now requiring C++20.

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• Resolves: TRI-1877

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge; the CMake logic now consistently enforces C++20 for default and stale-cache builds.

Summary

• Raises stale cached standards below 20 to the new minimum.
• Preserves valid user overrides requesting a standard higher than C++20.
• Fully addresses the previously reported stale-cache behavior.

_{Reviews (2) · Last reviewed commit: "fix: raise stale cached TRITON_MIN_CXX_S..."}