Fix two fmt uses that fail to compile under C++20

[Tobias-Fischer]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

Two fmt call sites compile today only because Open3D sets CMAKE_CXX_STANDARD 17. They break as soon as anything raises the standard to C++20 — which happens without any change to Open3D itself, since INTERFACE_COMPILE_FEATURES is a floor that propagates: linking one dependency whose imported targets declare cxx_std_20 raises every Open3D translation unit to C++20.

I hit this building against a packaged Filament whose CMake config declares cxx_std_20, but nothing here is specific to that; setting -DCMAKE_CXX_STANDARD=20 reproduces both.

1. TensorMap::ToString() passes a runtime std::string as a format string. Under C++20 fmt's format-string checking becomes consteval, so it is rejected:

cpp/open3d/t/geometry/TensorMap.cpp:167:27: error: call to consteval function
'fmt::fstring<const std::string &, std::string, std::string, std::string>::fstring<std::string, 0>'
is not a constant expression
note: read of non-constexpr variable 'tensor_format_str' is not allowed in a constant expression

fmt::runtime() is fmt's documented way to opt a runtime format string out of compile-time checking. It is correct under both standards, so this is arguably the right call regardless.

2. MetricParameters::ToString() formats a std::vector<float> without including fmt/ranges.h. It currently works only if that header arrives transitively:

$PREFIX/include/fmt/base.h:2310:45: error: implicit instantiation of undefined template
'fmt::detail::type_is_unformattable_for<std::vector<float>, char>'
note: in instantiation of function template specialization
'fmt::format<const std::vector<float> &, const unsigned long &>' requested here
   114 |         return fmt::format(

Both APIs exist in the bundled fmt — 9.1.0 on MSVC and 12.1.0 elsewhere — and fmt::runtime() has been available since fmt 8, so this does not raise the fmt floor.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style to my code. — verified with clang-format 18.1.8, matching python/requirements_style.txt.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test results here. — test results below.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

Two lines changed plus one include. No behaviour change: fmt::runtime() selects the runtime formatting path that C++17 already takes implicitly, and the added include only makes an existing dependency explicit.

Test results

Built Open3D 0.20.0 on osx-arm64 with both fixes, in a configuration where a dependency forces C++20 (confirmed via CMake's generated flags.make, -std=c++20). Both files compile and the build's packaging tests pass:

✔ python imports test passed!
✔ pip check passed!
✔ all tests passed!

Without these two changes the same build fails at TensorMap.cpp and then VtkUtils.cpp (via Geometry.h).

Scope and limitations

Tested on osx-arm64 with clang only. I have not built this on MSVC, where the bundled fmt is 9.1.0 rather than 12.1.0 — fmt::runtime and fmt/ranges.h both exist in 9.1.0, but I am relying on the API being present rather than having compiled it there.

I found these while making USE_SYSTEM_FILAMENT work (#7573). The two PRs are independent and can be reviewed and merged in either order.

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🤖 This pull request was written by Claude Code (Claude Opus 5), acting on behalf of @Tobias-Fischer, who has reviewed it.

https://claude.ai/code/session_017R4DS6whKCY8xGcA61kni2

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.