enable C++20 support

[aleflm]

PR intention

Needed for bip155 port.

Depends on: #1691

[coderabbitai]

Walkthrough

The PR raises the C++ standard to C++20 across build files and dependency packages, updates Boost to 1.83.0 (with hash), adjusts allocator public typedefs for C++20 compatibility, and fixes a genesis callback disconnect call in initialization.

Changes

Cohort / File(s)	Summary
Top-level CMake CMakeLists.txt, src/secp256k1/CMakeLists.txt	Bumped CMAKE_CXX_STANDARD from 17 to 20
Dependency build flags & versions depends/packages/bdb.mk, depends/packages/boost.mk, depends/packages/qt.mk, depends/packages/zeromq.mk	Changed package C++ flags to C++20; boost.mk also bumps Boost from 1.81.0 → 1.83.0 and updates SHA256
Allocator typedefs src/support/allocators/mt_pooled_secure.h, src/support/allocators/pooled_secure.h	Replaced inherited allocator typedefs (pointer, const_pointer, reference, const_reference) with explicit T*, const T*, T&, const T& aliases
Initialization fix src/init.cpp	Use &BlockNotifyGenesisWait when disconnecting the genesis wait callback to match the connected callback form

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

• Pay attention to Boost ABI/compatibility implications after version bump (depends/packages/boost.mk).
• Verify allocator typedef changes don't break containers or allocator-aware code expecting std::allocator traits.
• Confirm all build scripts and package recipes consistently use C++20 flags.

Poem

🐰 C++ twenty hops into sight,

Boost refreshed and typedefs made right,
Callbacks fixed with a tidy ampersand,
Builds leap forward across the land —
A rabbit’s cheer for the updated flight 🌿✨

Pre-merge checks and finishing touches

❌ Failed checks (1 warning, 1 inconclusive)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.
Description check	❓ Inconclusive	The PR description is incomplete. While it provides minimal intent ('Needed for bip155 port'), it lacks the mandatory 'PR intention' section detail and omits the optional 'Code changes brief' section explaining the architectural rationale.	Expand the description to explain why C++20 is needed for bip155 support and document any architectural implications of the upgrade.

✅ Passed checks (1 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'enable C++20 support' accurately and concisely summarizes the main change across all modified files: upgrading C++ standard from 17 to 20 in CMake and build configurations.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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📜 Recent review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between d795531 and e088c85.

📒 Files selected for processing (9)

• CMakeLists.txt (1 hunks)
• depends/packages/bdb.mk (1 hunks)
• depends/packages/boost.mk (2 hunks)
• depends/packages/qt.mk (1 hunks)
• depends/packages/zeromq.mk (1 hunks)
• src/init.cpp (1 hunks)
• src/secp256k1/CMakeLists.txt (1 hunks)
• src/support/allocators/mt_pooled_secure.h (1 hunks)
• src/support/allocators/pooled_secure.h (1 hunks)

🚧 Files skipped from review as they are similar to previous changes (6)

• CMakeLists.txt
• src/secp256k1/CMakeLists.txt
• src/init.cpp
• depends/packages/qt.mk
• depends/packages/zeromq.mk
• src/support/allocators/pooled_secure.h

🔇 Additional comments (4)

src/support/allocators/mt_pooled_secure.h (1)

24-27: LGTM! Correct C++20 allocator compatibility fix.

These explicit typedef definitions are necessary because std::allocator removed pointer, const_pointer, reference, and const_reference in C++20 (they were deprecated in C++17 per P0174R2). Custom allocators that inherit from std::allocator must now explicitly define these types if downstream code depends on them.

depends/packages/boost.mk (2)

24-24: Boost 1.83.0 officially supports C++20 compilation. Boost's 1.83.0 release (August 2023) is usable with C++20, with many libraries adopting or interoperating with C++20 facilities. Tested C++20-capable compilers include GCC 8+, Clang 11+, and recent MSVC versions.

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2-2: Verify Boost 1.83.0 hash is correct.

Boost 1.83.0 is a legitimate release (August 2023) with no reported security vulnerabilities. However, independently verify the SHA256 hash 6478edfe2f3305127cffe8caf73ea0176c53769f4bf1585be237eb30798c3b8e against the official Boost release archives to ensure correctness before merging.

depends/packages/bdb.mk (1)

12-12: BDB 4.8 C++ bindings cannot compile with -std=c++20 without compatibility patches

Berkeley DB 4.8's C++ API (db_cxx.h) uses std::auto_ptr and other deprecated C++ constructs that were removed in C++17. Building with -std=c++20 requires either:

• Keeping BDB compiled with an older standard (e.g., -std=c++11 or -std=gnu++14), or
• Patching the BDB sources to replace std::auto_ptr with std::unique_ptr and remove other deprecated constructs.

Confirm that any required compatibility patches have been applied to the BDB sources in depends/patches/ and that builds pass on all supported toolchains (Linux/Windows, gcc/clang/MSVC).

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