sx/issues/0154-sret-array-before-pointer-field.md

issue 0154 — null / --- assigned to a struct field over-stores when an enclosing target_type leaks

RESOLVED (fix landed). Root cause: null_literal / undef_literal were absent from the needs_target switch in lowerAssignment (src/ir/lower/stmt.zig), so obj.field = null (or = ---) did NOT set target_type to the field's type. While lowering a function body, target_type is set to the function's RETURN type (decl.zig:2691) for the whole body, so that leaked type reached constNull/constUndef (expr.zig:1788) and built a WHOLE-STRUCT-typed null. Emit then stored a struct-sized zeroinitializer through a GEP at the field's offset — an oversized store that overran the field's slot and clobbered the saved x29/x30, so the function ret'd to 0x0. Field order mattered only because a pointer field after an array field sits at a non-zero offset, pushing the over-store off the end of the alloca (pointer-first kept it in-bounds → silent but harmless). Manifested at -O0 (sx run JIT + sx build --opt 0); -O2 optimized the redundant stores away, masking it.

Fix: add .null_literal, .undef_literal to the needs_target switch (src/ir/lower/stmt.zig), so the field's type is resolved via fieldLvalueResolve and target_type is set to the field type — null builds an *i64-typed null (correct 8-byte store ptr null), not a struct.

Regression test: examples/types/0193-types-sret-array-before-pointer.sx.

Symptom

Returning a struct by value segfaults (read at 0x0) when a fixed-array field precedes a pointer field. Observed: Segmentation fault at address 0x0; expected: the struct's fields read back correctly.

Reproduction

#import "modules/std.sx";
S :: struct {
    arr: [2]u64;   // fixed-array field FIRST
    p:   *i64;     // pointer field AFTER the array
    n:   i64;
}
mk :: () -> S {
    s : S = ---;
    s.p = null;    // ← leaked return-type target_type → whole-struct null → over-store
    s.n = 0;
    return s;
}
main :: () -> i64 {
    s := mk();
    print("n {}\n", s.n);   // expected: n 0 ; actual: Segmentation fault at 0x0
    return 0;
}

Trigger matrix (aarch64-macos, -O0): array-before-pointer + by-value return → segfault; pointer-before-array → OK; array with no pointer → OK; by-value param pass → OK; local copy → OK. ⇒ the null/--- field-store value type, not the sret ABI.