Prevent byte relocation of non-trivial VLA elements

[thirtytwobits]

reserve() and shrink_to_fit() could pass live non-trivially-copyable elements to an allocator's reallocate, which may move the block by copying bytes. Address-sensitive elements then retained pointers into released storage, and element constructors/destructors were bypassed.

Gate the shared reallocation helper on std::is_trivially_copyable<value_type>. Other elements use the existing allocator-aware relocation and rollback from #215. Trivial storage and packed bool retain reallocation support. Non-trivial elements now require replacement storage for capacity changes, including when the underlying resource could resize in place; the public documentation explains reserving capacity up front for single-allocation resources.

Stacked on #215 (codex/issue-37-vla-partial-construction, base commit 38027ab). The implementation remains self-contained.

Tests were written, run against the unchanged implementation, and committed before the fix:

• 59fbcab: deterministic moving-resource regressions for reserve, shrink, implicit growth, address-sensitive elements, move-only/copy-fallback lifetimes, allocation and construction failures, trivial storage, packed bool, and allocators without reallocate.
• 90ed933: single-allocation resource behavior, plus explicit capacity setup in existing copy-assignment and fill-resize tests that previously relied on non-trivial in-place growth.
• dc8e9c4: production gate and documentation.

The reallocation suite centralizes numeric scenario inputs and derives sizes, payloads, packed capacities, and lifetime expectations from them. Static assertions protect the relocation, failure-position, bit-pattern, and numeric-range requirements; implicit resize derives its target from the capacity obtained after append.

Validation:

• Unchanged Fix VLA rollback after partial construction #215: 14 of 22 new tests failed with GCC C++14 Debug and Clang C++17 Debug; 10 of 18 failed with GCC C++14 ReleaseEP. The eight control cases passed. The additional single-allocation regression also failed against an isolated copy of the unchanged header.
• Fixed implementation: all 22 new tests pass with exceptions; all 18 applicable tests pass without exceptions.
• All 10 VLA suites passed GCC/Clang × C++14/17 × Debug/ReleaseEP (8 configurations). Reallocation and exception-safety suites passed GCC/Clang × C++14/17/20/23 × Debug/Release/DebugEP/ReleaseEP (32 configurations).
• Reallocation and exception-safety suites passed ASan/UBSan with leak detection in C++14 and C++17.
• Constant refactor: the focused suite passed GCC/Clang × C++14/17 × Debug/ReleaseEP. Two alternate constant configurations passed with and without exceptions, and 12 invalid configurations were rejected by their intended static assertions.
• Changed-code clang-format checks and git diff --check passed. Doxygen completed with the same 13 pre-existing warnings in type_traits_ext.hpp and unbounded_variant.hpp.

Closes #213 after this stack is merged into the default branch.

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused safety fix is consistent with allocator behavior and is covered across relevant element types and failure paths.

Review effort: Balanced
Findings: None

What changed in this PR

Prevents unsafe byte relocation of non-trivial VLA elements while retaining optimized reallocation for trivial and packed-bool storage.

Changes:

• Gates allocator reallocation on trivial copyability.
• Documents capacity behavior for single-allocation resources.
• Adds comprehensive relocation, failure, and lifetime tests.

File	Description
include/​cetl/​variable_length_array.hpp	Adds the safe reallocation gate and documentation.
cetlvast/​suites/​unittest/​test_variable_length_array_reallocation.cpp	Tests relocation safety and failure handling.
cetlvast/​suites/​unittest/​test_variable_length_array_general_allocation.cpp	Updates fixed-buffer tests and covers replacement-storage limits.
cetlvast/​suites/​unittest/​CMakeLists.txt	Registers the new test suite.

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