sx/issues/0126-array-arg-slice-generic-param-unbound.md

0126 — array arg at a []$T param leaves T unbound → LLVM emission panic

Symptom

Calling a generic function whose param is a slice of the type param — first :: (xs: []$T) -> T — with an ARRAY argument panics the compiler during emission instead of compiling (or diagnosing).

• Observed: panic: unresolved type reached LLVM emission — a type resolution failure was not diagnosed/aborted (src/backend/llvm/types.zig:175, via emitIndexGet in the monomorphized body).
• Expected: the array coerces to a slice at the []T param — the same promotion a CONCRETE []s64 param (and a []s64-annotated local) already performs — so T binds from the array's element type and the call compiles.

Passing an actual slice works (s : []s64 = a; first(s) prints the element); only the direct array spelling breaks, and only for generic slice params.

Reproduction

#import "modules/std.sx";

first :: (xs: []$T) -> T {
    return xs[0];
}

main :: () -> s32 {
    a : [3]s64 = ---;
    a[0] = 7; a[1] = 8; a[2] = 9;
    v := first(a);
    print("{}\n", v);
    return 0;
}

Observed at master 837b5d3: the panic above. With s : []s64 = a; first(s) it prints 7.

Investigation prompt

Root cause: extractTypeParam (src/ir/lower/generic.zig, the .slice_type_expr arm) only extracts when the ARG type is itself a .slice — an .array arg returns null, so buildTypeBindings leaves T unbound, monomorphizeFunction stamps the body with T → .unresolved, and nothing diagnoses before the emitter's sentinel tripwire fires (the declaration-time gate from ca5bd52 doesn't apply: this T IS bindable — the call-site inference is what failed).

Fix: mirror the existing array→slice param coercion in the binding extractor — in the .slice_type_expr arm, when the arg type is an .array, recurse on the array's ELEMENT type exactly as the .slice case does. Verify the lowered call then coerces the array arg to the mono's now-concrete []T param (the same array_to_slice the concrete path uses) — if not, the generic dispatch arg path needs the same promotion.

Out of scope, known gap (CHECKPOINT-MEM): string deliberately does not bind []$T — that case diagnoses "unknown type 'T'" and its story is deferred to the mem-stream phases.

Verification: the repro prints 7; the slice spelling still works; zig build && zig build test, bash tests/run_examples.sh green; pin the repro as a generics example.