refactor: modernize FuzzedDataProvider with if constexpr and safe casts #34396

1. 
   b-l-u-e commented at 10:41 pm on January 23, 2026: contributor

   This PR modernizes FuzzedDataProvider.h by resolving outstanding C++17 migration TODOs.

   1. Replace conditional with if constexpr in ConvertUnsignedToSigned

      • The TODO noted this should be changed once C++17 becomes mainstream

   2. Fix signed/unsigned comparison

      • Add explicit cast in comparison to avoid implicit conversions
      • Makes the type conversion explicit and safer

   3. Remove outdated TODO in PickValueInArray

      • The TODO suggested using static_assert, but this is not possible since std::initializer_list::size() is a runtime value when passed as a function parameter

   Motivation:

   These TODOs have been present since the code was written, waiting for C++14/C++17 adoption. Since Bitcoin Core now uses C++20, these modernizations are long overdue.

2. refactor: modernize FuzzedDataProvider with if constexpr and safe casts 4059457410
3. DrahtBot added the label Refactoring on Jan 23, 2026
4. 
   DrahtBot commented at 10:41 pm on January 23, 2026: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/34396.

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5. 
   l0rinc commented at 10:47 pm on January 23, 2026: contributor

   Please push these upstream to https://github.com/llvm/llvm-project/blob/main/compiler-rt/include/fuzzer/FuzzedDataProvider.h
6. 
   b-l-u-e commented at 11:06 pm on January 23, 2026: contributor

   Please push these upstream to https://github.com/llvm/llvm-project/blob/main/compiler-rt/include/fuzzer/FuzzedDataProvider.h

   Oh okay thanks…

7. b-l-u-e marked this as a draft on Jan 23, 2026
8. b-l-u-e closed this on Jan 23, 2026

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9. 
   b-l-u-e commented at 8:36 pm on January 24, 2026: contributor

   Opened a PR: https://github.com/llvm/llvm-project/pull/177794

Contributors
b-l-u-e DrahtBot l0rinc

Labels
Refactoring