Refactor numeric type handling and enhance conversion modules

[FrozenLemonTee]

No description provided.

[Copilot]

Pull request overview

This PR refactors numeric risk detection in the conversion subsystem to support underlying-type rep “bridging”, and adds new conversion modules to enable conversions between primitive<> wrappers and underlying types.

Changes:

• Refactors conversion.underlying numeric risk handling to operate via underlying-type traits (including built-in proxy selection via common_rep_traits).
• Adds new conversion modules for primitive↔primitive and primitive↔underlying casting (conversion.primitive, conversion.mixing) and exports them from conversion.
• Extends test support types and adds new test coverage for numeric risk bridging and primitive/mixed conversions.

Reviewed changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/conversion/underlying.cppm	Refactors numeric risk logic and adjusts cast API surface to work via underlying traits.
src/conversion/mixing.cppm	Adds overloads for casting between primitives and underlying types.
src/conversion/primitive.cppm	Adds primitive-to-primitive cast wrappers built on underlying conversions.
src/conversion/conversion.cppm	Re-exports the new conversion modules.
tests/basic/support/conversion_box_types.hpp	Adds bridged rep/box test types + underlying/common-rep trait specializations.
tests/basic/conversion/traits/test_numeric_risk.cpp	Adds tests for rep-bridge and built-in-proxy-bridge numeric risk detection.
tests/basic/conversion/primitive/test_primitive_casts.cpp	Adds tests for primitive-to-primitive casting behavior.
tests/basic/conversion/primitive_underlying/test_mixed_casts.cpp	Adds tests for underlying↔primitive mixed casting.
tests/basic/*/test_*.cpp (various)	Minor whitespace cleanup around includes.
.gitignore	Ignores cmake-build-* directories.

Comments suppressed due to low confidence (1)

src/conversion/underlying.cppm:207

• The rep-level helpers (unchecked_rep_cast/checked_rep_cast/saturating_rep_cast/...) are constrained on numeric_underlying_type for both DestRep and SrcRep. These helpers are invoked with underlying::traits<...>::rep_type, which can be a custom numeric rep (i.e., not an underlying_type), so conversions involving custom reps will fail to compile (or be uncallable) even when a static_cast between reps exists. Consider relaxing these templates to accept arbitrary rep types (e.g., just require statically_castable), and keep numeric risk checking gated on std_numeric; then perform numeric_underlying_risk checks at the underlying-type layer when you want bridged risk detection.

template <numeric_underlying_type DestRep, numeric_underlying_type SrcRep>
  requires statically_castable<DestRep, SrcRep>
constexpr auto unchecked_rep_cast(SrcRep value) noexcept
    -> std::remove_cvref_t<DestRep> {
  using dest_type = std::remove_cvref_t<DestRep>;
  return static_cast<dest_type>(value);
}

template <numeric_underlying_type DestRep, numeric_underlying_type SrcRep>
  requires statically_castable<DestRep, SrcRep>
constexpr auto checked_rep_cast(SrcRep value)
    -> cast_result<std::remove_cvref_t<DestRep>> {
  using dest_type = std::remove_cvref_t<DestRep>;
  using src_type = std::remove_cvref_t<SrcRep>;

  if constexpr (std_numeric<dest_type> && std_numeric<src_type>) {
    if (auto const kind = numeric_risk<dest_type>(value); kind.has_value()) {
      return std::unexpected(*kind);
    }
  }

  return static_cast<dest_type>(value);
}