build

[k2] implement visitors #5930

Job	Run time
macos/clang++/c++17	6m 16s
macos/clang++/c++17	5m 25s
buster/g++/c++17/asan=off/ubsan=off	17m 51s
buster/g++/c++17/asan=off/ubsan=off	18m 0s
focal/clang++/c++17/asan=off/ubsan=on	20m 38s
focal/clang++/c++17/asan=off/ubsan=on	21m 30s
focal/g++-10/c++20/asan=on/ubsan=off	21m 25s
focal/g++-10/c++20/asan=on/ubsan=off	21m 19s
jammy/g++/c++20/asan=on/ubsan=off	19m 19s
jammy/g++/c++20/asan=on/ubsan=off	19m 4s
	2h 50m 47s

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