[wgtmac]

LLVM 21.1.8's clang-scan-deps rejects digit separators in the #if expressions in compat/expected.h, even though ordinary Clang compilation accepts them. This breaks downstream CMake module dependency scanning with -format=p1689:

error: token is not a valid binary operator in a preprocessor subexpression

Use plain 201703L and 202002L literals in all nine language-version checks. The values and conditional behavior are unchanged.

Validation with LLVM 21.1.8:

• Reproduced nine dependency-scanning errors before the change in both C++23 and C++20 compatibility mode; the same default scanner succeeds after the change.
• Compiled and ran the existing expected_test.cc suite in both modes: 30 tests passed in each.
• pre-commit and git diff --check passed.

[Copilot]

🟢 Approval recommended

No active review issues are identified for the compatibility-header change.

No open issues 🎉

What changed in this PR

This PR makes the expected compatibility header compatible with LLVM 21.1.8 dependency scanning without changing its language-version conditions.

Changes:

• Removes digit separators from all nine preprocessor language-version checks.
• Preserves the C++17 and C++20 thresholds using 201703L and 202002L.

File	Description
src/​iceberg/​compat/​expected.h	Uses plain integer literals in preprocessor checks to avoid clang-scan-deps parsing errors.

_{Review effort: Balanced}

[zhjwpku]

LGTM