issue(0139): by-value self-referential type segfaults (typeSizeBytes recursion)

Discovered while testing metatype self-reference: a by-VALUE self-ref
(`payload = List`, not `*List`) infinite-loops typeSizeBytes → segfault
instead of a loud "infinite size" diagnostic. PRE-EXISTING — a hand-written
source enum `enum { node: Bad; leaf }` crashes identically, so it's a
general type-system gap (the comptime F5 by-value-rejection inherits the
fix). Filed per the IMPASSABLE rule; metatype checkpoint notes it.

current/CHECKPOINT-METATYPE.md

@@ -60,7 +60,14 @@ Pick any (independent):
   never `define()`d (hard error), use-before-define.
 
 ## Known issues
-None.
+- **issue 0139 — by-value self-reference segfaults** (`typeSizeBytes` infinite
+  recursion). A by-VALUE self-referential type (`payload = List` instead of
+  `*List`) stack-overflows instead of emitting a loud "infinite size" diagnostic.
+  PRE-EXISTING (a hand-written source enum `enum { node: Bad; leaf }` crashes
+  identically) — a general type-system gap, not specific to declare/define. Closes
+  the F5 "by-VALUE self-reference rejected" item for the comptime path once fixed.
+  Filed `issues/0139-byvalue-self-reference-segfault.md`; session stopped here per
+  the CLAUDE.md IMPASSABLE rule (do not work around / roll forward after filing).
 
 ## Log
 - **Self-reference done.** `declare(name)` + `preregisterForwardTypes` (forward

issues/0139-byvalue-self-reference-segfault.md

@@ -0,0 +1,59 @@
+# 0139 — by-value self-referential type segfaults (`typeSizeBytes` infinite recursion)
+
+**Symptom** — a type whose field/variant payload is ITSELF *by value* (not behind
+a pointer) crashes the compiler with a stack-overflow segfault instead of a loud
+"infinite size" diagnostic. Observed: `Segmentation fault` inside
+`src/ir/types.zig:typeSizeBytes` (unbounded self-recursion through the field
+loop). Expected: a clean compile error naming the offending type and suggesting
+`*Self`.
+
+**Pre-existing / scope** — NOT specific to the comptime `declare`/`define`
+metaprogramming; a hand-written SOURCE enum reproduces it identically. So the fix
+belongs in the general type-system size/layout path, and the comptime
+construction path (METATYPE F5) inherits the protection for free once it lands.
+
+**Reproduction** (source enum — no metaprogramming needed):
+
+```sx
+#import "modules/std.sx";
+
+Bad :: enum {
+    node: Bad;     // by-VALUE self-reference → infinite size
+    leaf;
+}
+
+main :: () -> i32 {
+    x : Bad = .leaf;
+    return 0;
+}
+```
+
+Same crash via the metaprogramming path (`#import "modules/std/meta.sx"`):
+
+```sx
+make_bad :: () -> Type {
+    h := declare("Bad");
+    return define(h, .enum(.{ variants = .[
+        EnumVariant.{ name = "node", payload = Bad },   // by-value self-ref
+        EnumVariant.{ name = "leaf", payload = void } ] }));
+}
+Bad :: make_bad();
+```
+
+**Investigation prompt** — A by-value self-referential (or mutually-recursive)
+aggregate has infinite size and must be rejected loudly, not recursed into
+forever. The crash is in `src/ir/types.zig:typeSizeBytes` (~line 736), which
+recurses into each struct/tagged-union field's type with no cycle guard; a field
+typed as its own (or an enclosing) nominal type recurses unboundedly →
+stack overflow. Fix: detect the cycle — walk the nominal-type dependency with a
+visited set (or a recursion-depth / on-stack-nominal guard), and when a nominal
+type reaches itself by value (no pointer indirection on the path), emit a
+diagnostic via `self.diagnostics.addFmt(.err, span, "type '{s}' is infinitely
+sized (it contains itself by value); use a pointer (`*{s}`) to break the cycle",
+…)` and return a sentinel size (e.g. 0 with the error already raised, or poison)
+rather than recursing. A pointer field (`*Bad`) breaks the cycle and must stay
+allowed (pointers have a fixed size and don't recurse into the pointee).
+Verification: run the repro above — expect the loud diagnostic + non-zero exit,
+NOT a segfault. Add an `examples/11xx-diagnostics-*` (or `issues/`) pin once the
+message is settled. This also closes METATYPE PLAN F5's "by-VALUE self-reference
+rejected" item for the comptime path.