sx/issues/0116-const-write-not-rejected.md

0116 — writes through module consts are not rejected (struct-const write bus-errors at runtime)

RESOLVED (2026-06-11, PLAN-CONST-AGG step 2). The assignment lowering rejects any target chain ROOTED at a constant — a const-flagged global (array consts, #run consts) or a module value const (struct consts incl.) — with cannot assign through constant 'X'. A deref along the chain (p.*) breaks the root (pointer writes are permanently unchecked — the documented pointer contract, specs.md §Pointer Types); a local shadowing the const name stays writable. Regression test: examples/1162-diagnostics-const-write-rejected.sx (struct field — the crash repro, array element, compound, bare scalar).

Symptom. Assigning through a module-level constant compiles silently. For a struct-literal const the store lands in read-only memory and the program crashes at runtime.

• Observed: compiles; Bus error at runtime on the store.
• Expected: compile-time diagnostic — cannot assign through constant 'WHITE'.

Reproduction

#import "modules/std.sx";

Color :: struct { r, g, b: s64; }
WHITE :: Color.{ r = 255, g = 255, b = 255 };

main :: () {
    WHITE.r = 0;     // compiles; bus error at runtime
    print("{}\n", WHITE.r);
}

(Copies are fine and stay fine: w := WHITE; w.r = 0; mutates the copy.)

Fix

Scheduled as step 2 of current/PLAN-CONST-AGG.md (aggregate consts): one rejection rule for assignment/compound-assignment targets whose ROOT identifier resolves to a module const (module_const_map author) or a const-flagged global (the array consts that plan introduces). Diagnose at the assignment site; pin the repro above as a diagnostics example.