Merge bitcoin/bitcoin#28349: build: Require C++20 compiler

[DashCoreAutoGuix]

Backports bitcoin#28349

Original commit: 3e69125

Backported from Bitcoin Core v0.27

This PR updates Dash Core to require a C++20 compiler, following Bitcoin Core's transition. The changes include:

• Updated build configuration to require C++20
• Updated documentation to reflect C++20 as the default standard
• Updated clang-format configuration for C++20
• Minor update to ax_cxx_compile_stdcxx.m4 for C++20 support

Note: Most of the C++20 changes were already present in Dash Core. This PR completes the transition by updating the remaining build system file.

Summary by CodeRabbit

• Chores
  • Updated compiler compatibility checks to temporarily allow certain older compilers to pass C++20 requirements.

[coderabbitai]

Walkthrough

The macro in build-aux/m4/ax_cxx_compile_stdcxx.m4 was updated to temporarily lower the required __cplusplus value for C++20 detection from 202002L to 201709L, enabling compatibility with compilers like g++-10 that report the lower value. No changes were made to exported or public declarations.

Changes

Cohort / File(s)	Change Summary
C++ Standard Detection Macro build-aux/m4/ax_cxx_compile_stdcxx.m4	Lowered the __cplusplus threshold in the C++20 test body from 202002L to 201709L to allow compilers reporting C++17 to pass as C++20, with a comment explaining this is a temporary patch for g++-10.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

---

📜 Recent review details

Configuration used: CodeRabbit UI
Review profile: ASSERTIVE
Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 9a35783 and ec42561.

📒 Files selected for processing (1)

• build-aux/m4/ax_cxx_compile_stdcxx.m4 (1 hunks)

🧰 Additional context used

📓 Path-based instructions (1)

**

⚙️ CodeRabbit Configuration File

**: # CodeRabbit AI Review Instructions for Dash Backports

Your Role

You are reviewing Bitcoin Core backports to Dash Core. Your ONLY job is to validate that the Dash commit faithfully represents the original Bitcoin commit with minimal, necessary adaptations.

Critical Validation Rules

1. File Operations Must Match (AUTO-REJECT if violated)

• If Bitcoin modifies an existing file → Dash MUST modify (not create new)
• If Bitcoin creates a new file → Dash creates
• If Bitcoin deletes a file → Dash deletes
• Common failure: Bitcoin modifies keys.txt, Dash creates new file with 58 keys

2. Size Ratio Check (80-150% of Bitcoin)

• Count functional lines changed (exclude comments/whitespace)
• Dash changes should be 80-150% of Bitcoin's size
• Red flag: 2-line Bitcoin fix becoming 150+ lines in Dash

3. No Scope Creep

• Reject if you see: "TODO:", "FIXME:", "while we're here", "also fix"
• No unrelated refactoring or style changes
• Only Bitcoin's intended changes + minimal Dash adaptations

4. Bitcoin-Specific Code Detection

• Auto-reject witness/segwit code: msg_wtxidrelay, MSG_WTX, witness imports
• Auto-reject RBF (replace-by-fee) functionality
• Note: PSBT is supported in Dash (don't flag)

5. Mandatory Adaptations Only

• bitcoin → dash in strings/paths
• Bitcoin → Dash in user-facing text
• Port numbers: 8332→9998 (RPC), 8333→9999 (P2P)
• Hardcoded test values specific to Dash
• No other changes unless absolutely required

6. Completeness Check

• All files changed in Bitcoin must be present
• Extra files need clear justification (Dash-specific compatibility)
• Missing files = incomplete backport

Review Process

1. First: Check file operations match exactly
2. Second: Calculate size ratio
3. Third: Scan for scope creep patterns
4. Fourth: Detect Bitcoin-specific code
5. Fifth: Verify all changes are minimal adaptations

Output Format

VALIDATION: [PASS/FAIL]

File Operatio...

Files:

• build-aux/m4/ax_cxx_compile_stdcxx.m4

🧠 Learnings (2)

📓 Common learnings

Learnt from: DashCoreAutoGuix
PR: DashCoreAutoGuix/dash#0
File: :0-0
Timestamp: 2025-07-28T22:03:12.364Z
Learning: During multiple verification attempts of Bitcoin Core commit 06d469c26b backport to Dash PR #566, DashCoreAutoGuix consistently identified scope creep in interface_usdt_utxocache.py where additional pruning test functionality was added beyond the original Bitcoin commit. The user provided comprehensive fixes including both scope creep removal and missing mempool test file additions, but couldn't push due to authentication restrictions. The scope creep fix was identified as the priority to resolve CI failures.

Learnt from: DashCoreAutoGuix
PR: DashCoreAutoGuix/dash#0
File: :0-0
Timestamp: 2025-07-28T20:34:29.061Z
Learning: During Dash backport verification of Bitcoin Core commit 06d469c26b, scope creep was detected when additional pruning test functionality was added to interface_usdt_utxocache.py beyond what was in the original Bitcoin commit. The fix involved removing the extra test block while maintaining the core compiler flag fixes for USDT compilation errors.

Learnt from: DashCoreAutoGuix
PR: DashCoreAutoGuix/dash#0
File: :0-0
Timestamp: 2025-07-28T19:54:21.426Z
Learning: In Dash backports from Bitcoin Core, including necessary compilation fixes (such as API compatibility changes like UniValue get_int() → getInt<int>()) alongside the core backport is standard and expected practice. These compatibility fixes ensure the backported code compiles in Dash's evolved codebase while preserving Bitcoin's original functionality and intent.

Learnt from: DashCoreAutoGuix
PR: DashCoreAutoGuix/dash#0
File: :0-0
Timestamp: 2025-07-29T21:29:32.827Z
Learning: DashCoreAutoGuix successfully fixed scope creep in Bitcoin Core commit fcdb39d3ee backport by removing the parse test case from src/test/uint256_tests.cpp that was not part of the original Bitcoin commit. The fix was implemented in commit 16748115ce and verified through range-diff analysis.

Learnt from: DashCoreAutoGuix
PR: DashCoreAutoGuix/dash#0
File: :0-0
Timestamp: 2025-07-29T17:13:35.087Z
Learning: When backporting Bitcoin Core commits that use Python's textwrap.dedent() function in test files, the textwrap import statement needs to be explicitly added if it's missing in the Dash test file.

Learnt from: DashCoreAutoGuix
PR: DashCoreAutoGuix/dash#0
File: :0-0
Timestamp: 2025-07-27T22:35:10.176Z
Learning: In Dash backports, src/dashbls files are vendored dependencies that should not be modified during Bitcoin Core backports unless there is specific justification. Unauthorized modifications to vendored dependencies should be removed to maintain code integrity.

Learnt from: DashCoreAutoGuix
PR: DashCoreAutoGuix/dash#0
File: :0-0
Timestamp: 2025-07-29T03:41:03.662Z
Learning: In Dash backports from Bitcoin Core, when converting function parameters from std::string to std::string_view, the find_value() method calls require explicit conversion back to std::string using std::string() wrapper, as find_value() expects std::string parameters in Dash's UniValue implementation.

Learnt from: DashCoreAutoGuix
PR: DashCoreAutoGuix/dash#0
File: :0-0
Timestamp: 2025-07-29T17:13:35.087Z
Learning: In Dash backports from Bitcoin Core, when the DIFFICULTY_ADJUSTMENT_INTERVAL constant is missing, it should be defined as 24 for Dash (different from Bitcoin's value), as seen in the getnetworkhashps RPC backport fix.

Learnt from: CR
PR: DashCoreAutoGuix/dash#0
File: CLAUDE.md:0-0
Timestamp: 2025-07-28T23:09:09.522Z
Learning: Applies to src/**/*.{cpp,h,cc,cxx,hpp} : C++20 codebase should be placed under src/

build-aux/m4/ax_cxx_compile_stdcxx.m4 (1)

Learnt from: CR
PR: DashCoreAutoGuix/dash#0
File: CLAUDE.md:0-0
Timestamp: 2025-07-28T23:09:09.522Z
Learning: Applies to src/**/*.{cpp,h,cc,cxx,hpp} : C++20 codebase should be placed under src/

🔇 Additional comments (1)

build-aux/m4/ax_cxx_compile_stdcxx.m4 (1)

986-986: LGTM - Compiler compatibility fix for C++20 detection.

The change correctly relaxes the __cplusplus version check to accommodate g++-10, which reports 201709L instead of the full C++20 value 202002L. This is a reasonable temporary compatibility measure for the C++20 transition.

Note: 201709L corresponds to C++17's initial value - you may want to verify this is the intended threshold for g++-10 compatibility.

VALIDATION: PASS

File Operations: MATCH (Bitcoin modifies existing file, Dash modifies same file)
Size Ratio: 100% (1 line change in both)
Scope Creep: NONE
Bitcoin-Specific Code: NONE
Completeness: COMPLETE

✨ Finishing Touches

🧪 Generate unit tests

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch backport-0.27-batch-544-pr-28349-new

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[DashCoreAutoGuix]

✅ Backport Verification Successful

No issues found! This backport is complete and faithful to the original Bitcoin change.

Original Bitcoin commit: 3e691258d8
Reviewed commit hash: ec42561fd6-verify-1753826185

✅ Analysis Details

Changes Verified ✅

• build-aux/m4/ax_cxx_compile_stdcxx.m4: C++20 requirement correctly applied
• configure.ac: C++20 compiler requirement already present
• depends/Makefile: CXX_STANDARD=c++20 already configured
• depends/README.md: Documentation updated to reference c++20
• depends/packages/qt.mk: Qt configured for c++2a (C++20)
• src/.clang-format: Standard updated to c++20
• src/test/fuzz/fuzz.h: C++20 pragma cleanup already applied
• src/test/fuzz/rpc.cpp: C++20 starts_with() already implemented
• src/util/trace.h: Variadic macro pragma cleanup already applied

Expected Differences ✅

• .github/workflows/ci.yml: Not applicable (Dash uses different CI)
• src/txrequest.cpp: Not present in Dash (Bitcoin-specific component)

CI Status ✅

All CI checks passing (0 failures out of 28 total checks)

This PR successfully implements the C++20 compiler requirement and associated code modernizations. All Bitcoin changes that are applicable to Dash have been correctly implemented. ✅

[DashCoreAutoGuix]

Verification Result: Logical Conflict Detected

This PR has a logical conflict with bitcoin#30228 which was already backported to Dash.

Analysis

Bitcoin PR bitcoin#28349 introduced a temporary workaround to allow GCC-10:

• Changed __cplusplus < 202002L to __cplusplus < 201709L in build-aux/m4/ax_cxx_compile_stdcxx.m4

Bitcoin PR bitcoin#30228 later reverted this workaround:

• Reverted back to __cplusplus < 202002L because Bitcoin now requires GCC 11+ (not GCC 10)
• See: build: Require C++20 compiler bitcoin/bitcoin#28349 (comment)

Dash Backport History

1. Commit ee7a6b48dd (in backport: merge bitcoin#24460, #27672, #27766, #28065, #30228, partial bitcoin#28349, #28579 (drop c++17 support) dashpay/dash#6380): Backported most of build: Require C++20 compiler bitcoin/bitcoin#28349 (including the GCC-10 workaround)
2. Commit ceedabb04b (in backport: merge bitcoin#24460, #27672, #27766, #28065, #30228, partial bitcoin#28349, #28579 (drop c++17 support) dashpay/dash#6380): Backported build: no-longer allow GCC-10 in C++20 check bitcoin/bitcoin#30228 (removed the GCC-10 workaround)
3. This PR Merge bitcoin/bitcoin#28349: build: Require C++20 compiler #632: Attempting to re-apply the GCC-10 workaround (conflicts with step 2)

Current State

• Dash currently requires GCC 11.1+ (see doc/dependencies.md)
• The GCC-10 workaround is not needed
• The codebase already has the correct state from build: no-longer allow GCC-10 in C++20 check bitcoin/bitcoin#30228

Recommendation

This PR should be closed as the changes it attempts to make:

1. Have already been applied (most changes in commit ee7a6b48dd)
2. Were subsequently superseded by build: no-longer allow GCC-10 in C++20 check bitcoin/bitcoin#30228 (commit ceedabb04b)
3. Would incorrectly revert build: no-longer allow GCC-10 in C++20 check bitcoin/bitcoin#30228

The backport of bitcoin#28349 is effectively complete - the only change that wasn't applied is the bit-field initialization in txrequest.cpp, which was correctly excluded because that file doesn't exist in Dash (it depends on bitcoin#19988).

[DashCoreAutoGuix]

Closing: This PR is superseded by bitcoin#30228 which was already backported in dashpay#6380.