check: reject an untyped array-literal borrow into a union slice variant (#18)

test/wcc/data/arrlit_slice_reject_union_untyped/case.ww

@@ -0,0 +1,21 @@
+//ww:error "not assignable"
+// #18 carrier: an UNTYPED-int array literal returned into a tagged-union SLICE
+// success variant ([]i32 | e). The literal defaults to [3]int, which is NOT
+// assignable to any variant ([]i32 wants i32 elements; e is i32), so BOTH stages
+// must REJECT at the assignability layer — harec tagged_select_subtype needs a
+// UNIQUE assignable member and the STORAGE_SLICE arm requires element identity
+// (ref/harec/src/types.c:702 + :1097). cstage rejects via type_assignable
+// returning 0 (cmd/wcc/type.c:344); wwstage's isassignable previously shape-
+// matched [3]int against the `e` (i32) scalar variant (both tagshape 0) and went
+// LENIENT (confident=false), so checkretassign short-circuited before the borrow
+// gate and SILENTLY ACCEPTED — a wwstage-only miscompile (cs≠ww). The fix makes
+// an array src with no element-matching variant a CONFIDENT reject in isassignable.
+// Sibling of #13 (the TYPED-literal leg, rejected by the borrow gate after it
+// passes assignability) and distinct from #17 (a legal array VARIABLE coercion).
+package main;
+type e = !i32;
+fn mk() ([]i32 | e) = { return [10, 20, 30]; };
+export fn main() i32 = {
+	let s: []i32 = mk()!;
+	return s[0];
+};