The metatype system can MINT tagged-union types (`make_enum`) and READ them
(`field_count`/`field_name`/`field_type`/`field_value`); `make_variant` is the
missing WRITE side — build a value of a minted tagged-union `E` whose active
variant is `idx`, carrying `payload`. Needed when the variant is selected at
RUNTIME but the union was synthesized at comptime so its labels can't be spelled
as a literal `.label(payload)` — e.g. the `race` result: an `inline for 0..N (i)`
arm constructs the i-th variant of a synthesized result with the winner's value.
Pure sx in modules/std/meta.sx (NOT a compiler builtin): a minted tagged-union is
laid out `{ i64 tag @0, payload @ size_of(i64) }` (the compiler fixes `tag_type`
to i64 for minted enums), so make_variant zeroes the value then writes the tag and
payload at those offsets — the same offsets the compiler's own enum_init/payload
ops use. The header comment documents the minted-only / i64-tag layout coupling.
Adversarially reviewed (SHIP): the offset-8 payload assumption is exactly the
compiler's enum layout with no alignment hazard (the payload is an alignment-1
`[N x i8]` array immediately after the i64 header); complex payloads (multi-field
struct, string fat pointer, 40-byte struct) round-trip. Locked by
examples/comptime/0650-comptime-make-variant.sx (heterogeneous + complex payloads,
runtime-index selection via the comptime `case` form). Suite green (822/0).
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