SFINAE to Concepts migration

[DennisOSRM]

Migration plan to replace SFINAE/detection idioms with C++20 concepts. Draft PR: #7529

SFINAE → Concepts migration plan

Overview

Replace ad-hoc SFINAE and detection-idiom patterns with C++20 concepts across the repository. This will make compile-time constraints explicit, simplify overload resolution, improve diagnostics and maintainability, and enable safer incremental refactors.

Goals

• Replace uses of std::enable_if, std::void_t, custom detection traits and similar SFINAE patterns with clear C++20 concepts and requires-clauses.
• Keep runtime behavior unchanged and preserve runtime polymorphism where required.
• Migrate incrementally with small, reviewable PRs and CI enforcement for C++20.

Scope (initial inventory)

Initial scan shows SFINAE / detection idiom uses in:

• include/util/graph_traits.hpp
• include/util/alias.hpp
• include/util/packed_vector.hpp
• include/util/indexed_data.hpp
• include/util/filtered_integer_range.hpp
• include/util/static_rtree.hpp
• include/util/packed_vector.hpp
• include/partitioner/* (cell_storage.hpp, multi_level_partition.hpp)
• include/extractor/node_data_container.hpp
• include/server/api/parameters_parser.hpp
• include/guidance/turn_data_container.hpp
• others discovered by repo-wide grep for enable_if / void_t / is_detected

Approach & Phases

1. Inventory & analysis

   • Run an automated scan (ripgrep/clang-query) to enumerate all SFINAE patterns and categorize them by pattern, file, and risk (low/medium/high).
   • Produce a spreadsheet or JSON mapping: file -> pattern -> instances -> suggested concept.

2. Design PoC / Policy

   • Formalize guidelines for when to prefer standard concepts (std::integral, std::derived_from, etc.) vs custom concepts.
   • Define a set of reusable concepts in include/concepts/ (or similar) and examples.
   • Complete two PoCs: RoutingAlgorithm concept (done) and GraphTraits -> concept.

3. CI and Tooling

   • Add CI job(s) compiling with -std=c++20 on multiple compilers (Clang/GCC) and platforms.
   • Add tests that assert critical concept constraints (static_asserts) and compile-examples.

4. Incremental migration

   • Create small PRs for low-risk modules first (util/*), then mid-risk (guidance/turn_data_container, server/api), then high-risk (partitioner, extractor).
   • Each PR must build on CI, include unit/regression checks and clear commit messages with rationale.

5. Cleanup & Documentation

   • Remove deprecated SFINAE helpers only after consumers migrated.
   • Update CONTRIBUTING.md with concept guidelines and examples.

Tasks and Milestones

• Inventory (sfinae-inventory)
• PoC: graph_traits -> concept (sfinae-poc-graph-traits)
• PoC: packed_vector -> concepts (sfinae-poc-packed-vector)
• Migrate util/* (sfinae-migrate-util)
• Migrate partitioner/extractor (sfinae-migrate-partitioner)
• Add CI concept checks (sfinae-ci)
• Documentation and examples (sfinae-docs)

Project Tracking

• Create a GitHub project board (repository-scoped) named: "SFINAE → Concepts migration" with columns: Backlog, In Progress, Review, Done.
• Track each task above as a card; link PRs and issues.

Automation & Helpers

• Provide a small scanning script (scripts/find_sfinae.sh) that uses ripgrep to find patterns and outputs JSON.
• Provide replacement snippets and templates for common transformations (enable_if -> requires, detection trait -> requires expression).
• Consider writing clang-tidy checks or clang-based refactor tooling for bulk changes if many mechanical replacements are required.

Risks & Mitigations

• Risk: single change causes many cascade compile errors. Mitigation: do per-module PRs and keep SFINAE helpers working as fallbacks until consumers are migrated.
• Risk: compiler differences in concept diagnostics. Mitigation: enforce CI across compilers and keep patch size small.
• Risk: behavioral regressions from incorrect replacements. Mitigation: rely on unit tests and acceptance tests before merge.

Acceptance criteria

• Each migrated module compiles under C++20 and passes existing tests.
• CI includes C++20 concept checks and is green for all PRs.
• A documented set of reusable concepts is available in-tree.

Next steps

• Finalize inventory and confirm the candidate list (sfinae-inventory).
• Start PoC on include/util/graph_traits.hpp (sfinae-poc-graph-traits).
• Create GitHub project board and add initial tasks/cards.

References

• Draft PR: chore: C++20 RoutingAlgorithm concept #7529

[DennisOSRM]

Created sub-issues for SFINAE -> Concepts migration:

• SFINAE -> Concepts: Inventory SFINAE usages #7531: SFINAE -> Concepts: Inventory SFINAE usages
• SFINAE -> Concepts: PoC: graph_traits to concepts #7532: SFINAE -> Concepts: PoC: graph_traits to concepts
• SFINAE -> Concepts: PoC: packed_vector to concepts #7533: SFINAE -> Concepts: PoC: packed_vector to concepts
• SFINAE -> Concepts: Migrate util/* SFINAE to concepts #7534: SFINAE -> Concepts: Migrate util/* SFINAE to concepts
• SFINAE -> Concepts: Migrate partitioner and extractor templates #7535: SFINAE -> Concepts: Migrate partitioner and extractor templates
• SFINAE -> Concepts: CI: Add C++20 concept checks #7536: SFINAE -> Concepts: CI: Add C++20 concept checks
• SFINAE -> Concepts: Docs: Contributing guide for concepts #7537: SFINAE -> Concepts: Docs: Contributing guide for concepts