In #188242, we replaced `PointerUnion`'s `PointerIntPair` storage with
`PunnedPointer<void*>`. The old formatters relied on the PIP synthetic
provider (LLDB) / `get_pointer_int_pair helper` (GDB) which no longer
work.
Instead, read raw bytes from `PunnedPointer` and compute the active tag
from template argument type alignments -- the same fixed-width encoding
the C++ implementation uses. When template arg enumeration is truncated
(e.g., function-local types in GDB), the formatters fall back to showing
a tag-stripped `void*` instead of silently misdecoding.
Alternatives that didn't work out:
- Adding a C++ helper (`getActiveMemberIdx`) callable from Python: gets
optimized out even with `__attribute__((used, noinline))`, and
expression evaluation fails for synthetic children.
- Using `isa`/`dyn_cast` checks from Python: requires expression
evaluation, which does not work for local types or synthetic children
without a frame context.
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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