docs: file issue 0190 (void-failable fall-through leaves error slot uninitialized)

issues/0190-void-failable-fallthrough-uninitialized-error-slot.md

@@ -0,0 +1,71 @@
+# 0190 — void failable (`-> !`) implicit fall-through leaves the error slot uninitialized
+
+**Status:** OPEN
+
+## Symptom
+
+A `-> !` (void failable) function that exits by **implicit fall-through**
+(no explicit `return;`) does not initialize its error-channel slot, so a
+caller (or `main`) reads a non-zero garbage tag and reports a phantom
+unhandled error.
+
+- Observed: `main :: () -> ! { print("ok\n"); }` prints `ok` then
+  `error: unhandled error reached main: error.` and exits **1**.
+- Expected: exit **0** (specs.md §11: "the exit code is `0` for void /
+  `-> !` success"). Adding an explicit trailing `return;` makes it exit 0.
+
+This is the silent-uninitialized-slot failure mode: the success path
+should write "no error" into the channel just like an explicit `return;`
+does, but the fall-through path skips it.
+
+## Reproduction
+
+```sx
+#import "modules/std.sx";
+
+main :: () -> ! {
+    print("ok\n");
+}
+```
+
+Run: `./zig-out/bin/sx run repro.sx` → prints `ok`, then
+`error: unhandled error reached main: error.`, exit 1 (should be 0).
+
+A non-`main` void failable shows the same uninitialized slot downstream:
+
+```sx
+#import "modules/std.sx";
+
+noop :: () -> ! { }                 // falls through, no `return;`
+main :: () {
+    noop() catch (e) { print("phantom: {}\n", e); }   // fires spuriously
+}
+```
+
+Workaround (confirms root cause): an explicit `return;` at the end of the
+`-> !` body initializes the slot and the phantom error disappears.
+
+## Investigation prompt
+
+The error channel for a `-> !` function is the last slot of the return
+aggregate (specs.md §12 ABI). An explicit `return;` lowers to a write of
+the "no error" sentinel into that slot; the **implicit fall-through** exit
+path (end of body with no `return`) apparently omits that write, leaving
+the slot whatever was on the stack.
+
+Likely area: the function-epilogue / failable-return lowering in
+`src/ir/lower/` (the path that synthesizes the implicit return for a
+body that falls off the end — search for where a void/`-> !` function's
+trailing fall-through is lowered, and where the error slot's "no error"
+sentinel is written on the explicit-`return;` path). The fix: the
+implicit fall-through of a failable function must initialize the error
+slot to "no error" exactly like `return;` does.
+
+Verification: the two repros above must exit 0 / not fire the catch;
+`examples/errors/1026-errors-failable-main.sx` (which currently passes
+only because it ends in `return;`) must keep passing. Add a regression
+example: a `-> !` function (and a `main :: () -> !`) that succeeds by
+fall-through with no explicit `return;`.
+
+(Found by adversarial review during the tuple-syntax-cutover docs pass,
+commit `989e18b7`. Pre-existing — independent of the tuple change.)