sx/issues/0162-comptime-run-returning-optional-aggregate.md

0162 — #run returning an optional aggregate fails the comptime VM reg→value bridge

RESOLVED. Root cause: the comptime VM's regToValue bridge (src/ir/comptime_vm.zig) had no .optional arm, so any OPTIONAL-typed #run result hit the "aggregate shape not bridged yet" bail. Fix: added an .optional arm that reads the has_value flag (at offset sizeof(child)), bridges the payload recursively into a { payload, i1=true } aggregate when set, and yields .null_val (→ zero {T, i1}) when clear or the bare null sentinel; plus a matching serialize arm in serializeAggregateValue (src/ir/emit_llvm.zig). Pointer/?Closure/?Protocol-child optionals and array-payload aggregates bail loudly (out of scope, not silent). Regression test: examples/comptime/0643-comptime-run-optional-aggregate.sx (present ?T, present ?i64, null ?i64). Verified by 3 adversarial reviews.

Symptom

A #run (or comptime const init) whose function returns an OPTIONAL value (?T, ?i64, any optional) fails comptime evaluation with:

error: comptime init of 'X' failed: reg→value: aggregate shape not bridged yet

A non-optional return of the same type works. This is a pre-existing limitation in the comptime VM's register→value bridge for optional-typed results; it is orthogonal to issue 0160 (it reproduces for a value-init optional with no struct literal anywhere, and for a scalar optional ?i64).

Reproduction

#import "modules/std.sx";
T :: struct { a: i64 = 0; }
mk  :: () -> ?T   { t : T = .{ a = 7 }; return t; }
mk2 :: () -> ?i64 { return 5; }

X :: #run mk();    // error: reg→value: aggregate shape not bridged yet
Y :: #run mk2();   // same class of failure
main :: () { print("ok\n"); }

Baseline that WORKS: Z :: #run (() -> T { return .{ a = 7 }; })(); (non-optional).

Investigation prompt

src/ir/comptime_vm.zig — the reg→value bridge (search "aggregate shape not bridged" / regToValue) handles scalars/structs/slices but bails on an OPTIONAL-typed result. An optional is {payload, has_value} (or a pointer for ?*T / a sentinel for ?Closure); the bridge needs to read the has_value flag and, when set, bridge the payload as its child type (recursively), producing a Value optional — and a null optional when clear. Add the .optional arm to the reg→value bridge (mirror the value→reg direction, which already builds optionals — see makeStringList/writeField optional handling). Verify with the repro (expect X/Y to evaluate, main prints ok). Add a examples/comptime/06xx-... regression.