sx/issues/0053-comptime-pack-spread-into-any-slice.md

FIXED via the xx <pack> bridge (the preferred fix below), not by changing

RESOLVED. Spreading a comptime pack into a []Any/[]P parameter via xx args previously hit the pack-as-value diagnostic ("pack has no runtime value") before xx was considered, so no slice was ever materialized. Fixed by intercepting xx <pack> with a slice target in the unary_op .xx arm of lowerExpr (src/ir/lower/expr.zig) before lowering the operand, calling lowerPackToSlice (src/ir/lower/pack.zig:167) to box/erase each element into a runtime [N]elem → []elem. Covered by regression test examples/0537-packs-pack-xx-to-slice.sx. the ..args spread. xx args with a slice target now materializes the pack into a runtime []Any/[]P — see examples/204-pack-xx-to-slice.sx. lowerXX/the unary-op arm intercepts xx <pack> before the pack-as-value check and calls the new lowerPackToSlice (src/ir/lower.zig). The bare ..args spread into a non-variadic []Any param is still unsupported (use xx args); left as-is.

Symptom

Spreading a comptime pack ..$args into a []Any parameter — f(..args) where f takes items: []Any — fails LLVM verification:

LLVM verification failed: Incorrect number of arguments passed to called function!
  %call = call i64 @log_count(ptr %0, { ptr, i64 }, { ptr, i64 }, double ...)

The spread passes the pack's N elements as N separate positional args instead of materialising a single []Any slice for the one items parameter.

Reproduction

#import "modules/std.sx";
log_count :: (items: []Any) -> i64 { return items.len; }
forward :: (..$args) -> i64 { return log_count(..args); }
main :: () -> i32 { print("{}\n", forward(1, "hi", 2.5)); return 0; }

Expected: 3 (the pack spreads into the []Any slice, like calling log_count(1, "hi", 2.5) against a []Any variadic would).

Preferred fix — xx args (pack → slice materialization)

Rather than make the splat-y ..args spread collapse into a single slice arg, the cleaner spelling is an xx cast, which already means "erase/convert to the expected type":

forward :: (..$args) -> i64 { return log_count(xx args); }   // target: []Any

xx args (target-typed) should materialize the pack into the expected slice:

• target []Any → box each pack element to Any, build [N]Any → []Any;
• target []P → xx-erase each element to the protocol P, build [N]P → []P (reuse the slice-of-protocol erasure landed in packVariadicCallArgs, issue 0052).

This reuses the existing xx/protocol machinery, reads naturally, and keeps ..xs reserved for true spreads into pack/variadic callees.

Currently xx args errors ("pack 'args' has no runtime value") because the Step 2.7 pack-as-value check fires on the bare args operand before xx is considered. The fix: in xx (unary_op .xx) lowering, intercept a pack operand before the pack-as-value diagnostic and, when the target type is a slice, materialize as above.

Workaround today

Declare the forwarder as the slice variadic instead of a pack — then it's already a runtime []Any and forwards directly:

forward :: (..args: []Any) -> i64 { return log_count(args); }   // works -> 3

This is what examples/162-pack-bare-args.sx demonstrates.

Verification

After the fix, log_count(xx args) (and the original ..args form, if also fixed) should print 3 and pass sx ir LLVM verification.