The original code is a very precise heuristics to detect MSYS/Cygwin environment cmake (i.e. the one which only understands POSIX paths and doesn't offer VS generators, https://packages.msys2.org/base/cmake).

What you try to detect/prevent now is cmake from the windows environment offered by msys2 (i.e. probably offering VS generators but somewhat broken around invoking RC for the Ninja generator,
https://packages.msys2.org/base/mingw-w64-cmake). This is a different problem, and the heuristics isn't significant at all, for positives and for negatives.

Not fixed by the /msys2/ heuristics:
microsoft/vcpkg#47171: Windows cmake, C:/msys64/ucrt64/bin/cmake.exe
microsoft/vcpkg#46391: Windows cmake, C:/msys64/clang64/bin/cmake.exe
microsoft/vcpkg#46188: Windows cmake, C:/msys64/ucrt64/bin/cmake.exe
microsoft/vcpkg#44155,
microsoft/vcpkg#44154: Windows cmake, C:/msys64/mingw64/bin/cmake.exe
microsoft/vcpkg#47135: Windows cmake, F:/msys64/ucrt64/bin/cmake.exe

The real common thing is that paths end in <mingw-env>/bin,
https://www.msys2.org/docs/environments/

Not fixed by the /msys2/ heuristics:

Given that the title mentions /msys64/ and the paths also contain that string, you could have just pointed out that I had a typo in my code...

I think it's more efficient to search for /msys64/ because that requires only one comparison in contrast to 5 if I only considered the end of the path (the function is called cheap_is_acceptable after all). Furthermore, I'd consider it very likely that a cmake executable under the path /msys64/ belongs in fact to msys2.

What you try to detect/prevent now is cmake from the windows environment offered by msys2 (i.e. probably offering VS generators but somewhat broken around invoking RC for the Ninja generator,
https://packages.msys2.org/base/mingw-w64-cmake).

Judging by the comment on line 400, it's also intended to avoid using msys2's cmake.

Judging by the comment on line 400, it's also intended to avoid using msys2's cmake.

The comment says "version can't handle 'C:' in paths", but this is true only for /usr/bin/cmake.exe and /cygwin/bin/cmake.exe.
The windows environment cmakes (/ucrt64/bin/cmake.exe, /clang64/bin/cmake.exe, /mingw64/bin/cmake.exe) do handle C:.

I think it's more efficient to search for /msys64/ because that requires only one comparison in contrast to 5 if I only considered the end of the path (the function is called cheap_is_acceptable after all).

I think it is still extremely cheap to explicitly check the small set of known short strings at the end of the path.
At least in contrast to invoking the executable.

.. and path.find is a search operation, path.ends_with is a trivial comparison.

Is there anything that distinguishes the problematic CMakes in the --version output, since we're running that to check the version already anyway.

Is there anything that distinguishes the problematic CMakes in the --version output, since we're running that to check the version already anyway.

Quick check:

Not sure if the msvc suffix is robust enough:
official (windows): cmake version 3.31.6-msvc
msys2 (ucrt64): cmake version 4.1.0

Alternatively, I can point at the generator list from cmake -G, in particular the default generator marked by *:
official (windows): * Visual Studio 17 2022 ...
msys2 (ucrt64): * Ninja ...
(msys2 (msys runtime): No Visual Studio generators!)

There is also cmake --system-information but it is slow on Windows

Not sure if the msvc suffix is robust enough:
official (windows): cmake version 3.31.6-msvc
msys2 (ucrt64): cmake version 4.1.0

Unfortunately not:

C:\Program Files\Microsoft Visual Studio\2022\Enterprise>where cmake
C:\Program Files\CMake\bin\cmake.exe
C:\Program Files\Microsoft Visual Studio\2022\Enterprise\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin\cmake.exe

C:\Program Files\Microsoft Visual Studio\2022\Enterprise>cmake --version
cmake version 4.1.1

CMake suite maintained and supported by Kitware (kitware.com/cmake).

C:\Program Files\Microsoft Visual Studio\2022\Enterprise>

IMO it is sufficient and cheap to determine the parent dir of bin/cmake.exe, and to match it against the fairly stable list of msys environments (usr,mingw32,mingw64,ucrt64,clang64,clangarm64).