lang F1 2.6: pack-index edge cases (runtime-index error, comptime OOB)

Per locked Decision 1 a pack is comptime-only with no runtime value, so xs[i]
is valid only for a comptime index. lowerIndexExpr now emits a clear error
("pack <p> must be indexed by a compile-time constant ...") for a runtime
index, instead of the confusing "unresolved <p>" the slice-index fall-through
produced. diagPackIndexOOB switched from int-literal-only to comptimeIndexOf so
an inline-for cursor that goes out of bounds is also caught.

Repurposed examples/163-pack-runtime-index.sx (was aspirational: expected
runtime indexing to materialise a []Any slice and print 4, contradicting
Decision 1) into the runtime-index error test. Comptime + OOB cases already
covered by examples/199/200/161. 236 examples + unit green.

examples/163-pack-runtime-index.sx

@@ -1,17 +1,13 @@
-// Variadic heterogeneous type packs — follow-up #4 (runtime
+// Variadic heterogeneous type packs — Step 2.6: indexing a pack with a
-// pack indexing).
+// RUNTIME index is a compile error.
 //
-// `args[<literal>]` substitutes through `pack_arg_nodes` (step
+// Per locked Decision 1, a pack is comptime-only and has NO runtime
-// 2a.B). But `args[<runtime_int>]` — a loop counter, an
+// representation — so `args[i]` is valid only when `i` is a compile-time
-// expression — falls through to the standard slice-indexing
+// constant (a literal, or an `inline for` cursor). A runtime index (here a
-// path, which fails because the pack-mono doesn't materialise
+// `while`-loop counter) must produce a clear diagnostic, not the confusing
-// the `args` slice. Output today: "unresolved 'args'".
+// "unresolved 'args'" the slice-index fall-through used to give. To walk a
-//
+// pack, use `inline for 0..args.len (i) { ... }`, which unrolls so each
-// Pairs with follow-up #3: the same `[]Any` slice
+// `args[i]` is a comptime index.
-// materialisation handles both `args` bare and `args[i]`
-// runtime. Element type becomes `Any` (lossy on per-position
-// types — that's the inherent trade-off of runtime indexing
-// into a heterogeneous pack).
 
 #import "modules/std.sx";
 
@@ -19,9 +15,7 @@ count_anys :: (..$args) -> s64 {
     total : s64 = 0;
     i : s64 = 0;
     while i < args.len {
-        // Runtime index — should resolve through the
+        x : Any = args[i];   // ERROR: runtime index into a comptime-only pack
-        // materialised slice once #3+#4 lands.
-        x : Any = args[i];
         _ = x;
         total = total + 1;
         i = i + 1;

src/ir/lower.zig

@@ -4797,6 +4797,21 @@ pub const Lowering = struct {
         if (self.diagPackIndexOOB(ie)) {
             return self.builder.constInt(0, .s64);
         }
+        // Runtime index into a comptime-only pack (Decision 1): a pack has no
+        // runtime representation, so the index must be a compile-time constant.
+        // A runtime index is a hard error — clearer than the "unresolved
+        // '<pack>'" the slice-index fall-through would otherwise produce.
+        if (self.pack_param_count) |ppc| {
+            if (ie.object.data == .identifier) {
+                const pname = ie.object.data.identifier.name;
+                if (ppc.contains(pname) and self.comptimeIndexOf(ie.index) == null) {
+                    if (self.diagnostics) |diags| {
+                        diags.addFmt(.err, ie.index.span, "pack '{s}' must be indexed by a compile-time constant — a pack is comptime-only and has no runtime value", .{pname});
+                    }
+                    return self.builder.constInt(0, .s64);
+                }
+            }
+        }
         const obj = self.lowerExpr(ie.object);
         const idx = self.lowerExpr(ie.index);
         // Infer element type from the object's slice/array type
@@ -4815,8 +4830,10 @@ pub const Lowering = struct {
         if (ie.object.data != .identifier) return false;
         const pack_name = ie.object.data.identifier.name;
         const n = ppc.get(pack_name) orelse return false;
-        if (ie.index.data != .int_literal) return false;
+        // Any comptime index (int literal or a comptime-constant cursor) that's
-        const raw: i64 = ie.index.data.int_literal.value;
+        // out of range — runtime indices are handled by the caller's
+        // must-be-comptime check.
+        const raw: i64 = self.comptimeIndexOf(ie.index) orelse return false;
         if (raw >= 0 and @as(u32, @intCast(raw)) < n) return false;
         if (self.diagnostics) |diags| {
             diags.addFmt(.err, ie.index.span, "pack index {} out of bounds: '{s}' has {} element{s}", .{

tests/expected/163-pack-runtime-index.exit

@@ -1 +1 @@
-0
+1

tests/expected/163-pack-runtime-index.txt

@@ -1 +1,5 @@
-4
+error: pack 'args' must be indexed by a compile-time constant — a pack is comptime-only and has no runtime value
+  --> /Users/agra/projects/sx/examples/163-pack-runtime-index.sx:18:24
+   |
+18 |         x : Any = args[i];   // ERROR: runtime index into a comptime-only pack
+   |                        ^