Update C++ standard to version 20

[agarny]

Fixes #1379.

[Copilot]

Pull request overview

This PR updates libCellML’s build configuration and implementation code to require and take advantage of C++20, addressing issue #1379.

Changes:

• Bump the core library and related targets (tests + JS bindings) to compile with C++20.
• Modernise container/algorithm usage across the codebase (e.g., std::ranges::*, contains, starts_with, std::erase(_if)).
• Simplify and modernise some internal data structures (e.g., cached equivalence keying in AnalyserModel).

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/api_headers/api_header_test.in.cmake	Update API header inclusion test target to C++20.
src/CMakeLists.txt	Raise exported compile features for cellml and cellml_debug_utilities to cxx_std_20.
src/bindings/javascript/CMakeLists.txt	Update JS bindings target to C++20 to match core library requirement.
src/variable.cpp	Switch equivalent-variable searches/cleanup to C++20 ranges + std::erase_if.
src/validator.cpp	Replace various std::* algorithms/string-prefix checks with C++20 ranges + starts_with/contains/try_emplace.
src/utilities.cpp	Modernise helpers using contains and std::ranges::all_of, plus minor construction/emplace cleanups.
src/units.cpp	Move unit lookup to std::ranges::find_if.
src/printer.cpp	Use braced pair construction and std::ranges::find_if/find for map building.
src/parser.cpp	Canonicalise connection/variable pairs and use std::ranges::find for uniqueness checks.
src/model.cpp	Convert unit/import requirement lookups to std::ranges::*.
src/importer.cpp	Use contains, emplace, and iterator-based access to simplify library/import tracking.
src/generatorvariabletracker.cpp	Use contains when tracking analyser variables.
src/generator.cpp	Replace manual erase/find patterns with std::erase and std::ranges::find.
src/componententity.cpp	Convert component lookup to std::ranges::find_if.
src/component.cpp	Convert variable/reset lookup to std::ranges::find_if.
src/annotator.cpp	Use emplace/contains to simplify ID list maintenance.
src/analysermodel_p.h	Replace custom key struct with std::pair<uintptr_t, uintptr_t> and a hash functor.
src/analysermodel.cpp	Use std::minmax and the new key type for cached equivalence lookup.
src/analyserexternalvariable.cpp	Convert dependency searches to std::ranges::find_if.
src/analyserequation.cpp	Use std::erase_if for staging dependency removal.
src/analyser.cpp	Broad conversion from classic algorithms to ranges + container helpers (contains, std::erase(_if)).

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[hsorby]

In this PR all I am really only expecting to see is the changes to the two CMakeLists.txt files updating to C++ 20 standard. I don’t see the other changes that take advantage of this upgrade as part of this PR. The take advantage of the C++ 20 standard changes belong in a separate PR.