lib/io: empty() stream; wcc/cgen: #129 sretretsize + #130 global tagged-field store

io.empty (discard+EOF stream, ref/hare/io/empty.ha:4-17) — needed by getopt's
two-pass printusage width measurement. Diverges from Hare's `const empty: *stream`:
a `let _empty_vt` + `fn empty()` that wires the fn-ptr slots per call, because
const-init of a vtable struct with fn-ptr fields is blocked (#118, ruled accept).

Co-discovered while making empty() byte-identical across stages: three
wwstage-only cgen fixes (cstage was already correct; wwstage aligned down):
- #129 sretretsize: consult the same-module pointer-alias before structlookup's
  any-module struct fallback (io.stream = *vtable was mis-sized as memio's 56B
  struct -> spurious sret save).
- #129 callsretsize: swap curmod to the callee's module before sret-size
  classification (cross-module callee context).
- #130 cgassign global-struct tagged-union field store: add the missing arm
  (was a 1-word store) mirroring cstage cgen.c:4893-4912.

The three are inseparable from io.empty here — splitting them out leaves a
divergent-asm intermediate (993/995 red), so they ride one commit per the
one-class gate-repair carve-out (#133-expanded precedent). Regenerates the
embedded combined.ww; 989_lib_byteid pins bufio + fmt graduated to M_ID.
(cgenexpr.ww fix-3 inline comment cites the #129 cluster; narrow to #130 on
next touch to avoid a regen for a comment.)

lib/io/io.ww

@@ -6,12 +6,12 @@
 // tagged unions instead of errno-style integer sentinels.
 //
 // This file owns the eof / underread variant tags; lib/io/stream.ww
-// owns the `vtable` + `stream` + read/write/close dispatchers, and
-// lib/io/types.ww owns the error union, mode/whence enums, and the
-// reader/writer/closer fn-type aliases. #94 fold-eFinal collapsed the
-// pre-vtable `stream` struct + `closed` tag into the single vtable
-// surface; the dispatchers are read/write/close (over `stream`),
-// final over `handle` at io fold-2 (#5).
+// owns the `vtable` + `stream` + read/write/close dispatchers and the
+// [[empty]] singleton, and lib/io/types.ww owns the error union,
+// mode/whence enums, and the reader/writer/closer fn-type aliases.
+// #94 fold-eFinal collapsed the pre-vtable `stream` struct + `closed`
+// tag into the single vtable surface; the dispatchers are read/write/
+// close (over `stream`), final over `handle` at io fold-2 (#5).
 package io;
 
 // eof — read past the end of the stream. Hare uses the `done`

lib/io/stream.ww

@@ -1,7 +1,7 @@
 // stream — Hare-shaped vtable surface. Project #94 fold-eFinal.
 //
 // The single io stream surface (the fold-eFinal collapse retired the
-// pre-vtable `stream` struct + `closed` tag). Three exports:
+// pre-vtable `stream` struct + `closed` tag). Exports:
 //
 //   vtable     a struct of optional fn-pointer slots — reader/writer/
 //              closer per ref/hare/io/stream.ha:36-42. Hare spells the
@@ -21,6 +21,9 @@
 //              public `read`/`write`/`close`; ww has no `handle` yet
 //              (io fold-2, #5), so the dispatchers ARE the public
 //              surface and grow the `handle` match when #5 lands.
+//   empty      the discard+EOF stream (ref/hare/io/empty.ha:13). Lives
+//              here rather than io.ww so that 900_stdlib can compile
+//              io.ww standalone (io.ww has no cross-file type refs).
 //
 // Deferrals (drew-signed): `seeker` lands with io fold-2 (#5) once `off`
 // + `whence` plug into the signature. Hare's `?`-propagating `close`
@@ -183,3 +186,37 @@ export fn seek(h: handle, off: off, w: whence) (off | error) = {
 export fn tell(h: handle) (off | error) = {
 	return seek(h, 0, whence.CUR);
 };
+
+// ---- empty stream -------------------------------------------------------
+//
+// ref/hare/io/empty.ha:4-17.
+//
+// Hare uses `const _empty_vt: vtable = { ... }` + `const empty: *stream`.
+// ww can't const-init a vtable struct with fn-ptr fields (#118), so the
+// vtable is a module-level `let` and [[empty]] is a function that wires
+// the fn-ptr slots on every call and returns the stream pointer.
+// Single-assignment on the same words: idempotent under re-entry.
+// Lives in stream.ww (not io.ww) so that 900_stdlib can compile io.ww
+// standalone without referencing the cross-file vtable/reader/writer types.
+
+fn _empty_read(s: stream, buf: []u8) (size | eof | error) = {
+	let e: eof;
+	return e;
+};
+
+fn _empty_write(s: stream, buf: []u8) (size | error) = {
+	return buf.len: size;
+};
+
+let _empty_vt: vtable;
+
+// empty — a stream that discards all writes (returning their size) and
+// returns EOF on every read. Mirrors ref/hare/io/empty.ha:13.
+// #118: const vtable init with fn-ptr fields is unwired (emit_struct_data
+// needs a two-pass reloc extension, node_fnptr_sym reusable). Using a
+// mutable let + per-call wiring until #118 lands.
+export fn empty() stream = {
+	_empty_vt.reader = (&_empty_read): *reader;
+	_empty_vt.writer = (&_empty_write): *writer;
+	return &_empty_vt;
+};

selfhost/cmd/w6c/main.combined.ww

@@ -15470,12 +15470,12 @@ export fn checkfile(c: *checker, file: *node) void = {
 // tagged unions instead of errno-style integer sentinels.
 //
 // This file owns the eof / underread variant tags; lib/io/stream.ww
-// owns the `vtable` + `stream` + read/write/close dispatchers, and
-// lib/io/types.ww owns the error union, mode/whence enums, and the
-// reader/writer/closer fn-type aliases. #94 fold-eFinal collapsed the
-// pre-vtable `stream` struct + `closed` tag into the single vtable
-// surface; the dispatchers are read/write/close (over `stream`),
-// final over `handle` at io fold-2 (#5).
+// owns the `vtable` + `stream` + read/write/close dispatchers and the
+// [[empty]] singleton, and lib/io/types.ww owns the error union,
+// mode/whence enums, and the reader/writer/closer fn-type aliases.
+// #94 fold-eFinal collapsed the pre-vtable `stream` struct + `closed`
+// tag into the single vtable surface; the dispatchers are read/write/
+// close (over `stream`), final over `handle` at io fold-2 (#5).
 package io;
 
 // eof — read past the end of the stream. Hare uses the `done`
@@ -15492,7 +15492,7 @@ export type underread = !i32;
 // stream — Hare-shaped vtable surface. Project #94 fold-eFinal.
 //
 // The single io stream surface (the fold-eFinal collapse retired the
-// pre-vtable `stream` struct + `closed` tag). Three exports:
+// pre-vtable `stream` struct + `closed` tag). Exports:
 //
 //   vtable     a struct of optional fn-pointer slots — reader/writer/
 //              closer per ref/hare/io/stream.ha:36-42. Hare spells the
@@ -15512,6 +15512,9 @@ export type underread = !i32;
 //              public `read`/`write`/`close`; ww has no `handle` yet
 //              (io fold-2, #5), so the dispatchers ARE the public
 //              surface and grow the `handle` match when #5 lands.
+//   empty      the discard+EOF stream (ref/hare/io/empty.ha:13). Lives
+//              here rather than io.ww so that 900_stdlib can compile
+//              io.ww standalone (io.ww has no cross-file type refs).
 //
 // Deferrals (drew-signed): `seeker` lands with io fold-2 (#5) once `off`
 // + `whence` plug into the signature. Hare's `?`-propagating `close`
@@ -15675,6 +15678,40 @@ export fn tell(h: handle) (off | error) = {
 	return seek(h, 0, whence.CUR);
 };
 
+// ---- empty stream -------------------------------------------------------
+//
+// ref/hare/io/empty.ha:4-17.
+//
+// Hare uses `const _empty_vt: vtable = { ... }` + `const empty: *stream`.
+// ww can't const-init a vtable struct with fn-ptr fields (#118), so the
+// vtable is a module-level `let` and [[empty]] is a function that wires
+// the fn-ptr slots on every call and returns the stream pointer.
+// Single-assignment on the same words: idempotent under re-entry.
+// Lives in stream.ww (not io.ww) so that 900_stdlib can compile io.ww
+// standalone without referencing the cross-file vtable/reader/writer types.
+
+fn _empty_read(s: stream, buf: []u8) (size | eof | error) = {
+	let e: eof;
+	return e;
+};
+
+fn _empty_write(s: stream, buf: []u8) (size | error) = {
+	return buf.len: size;
+};
+
+let _empty_vt: vtable;
+
+// empty — a stream that discards all writes (returning their size) and
+// returns EOF on every read. Mirrors ref/hare/io/empty.ha:13.
+// #118: const vtable init with fn-ptr fields is unwired (emit_struct_data
+// needs a two-pass reloc extension, node_fnptr_sym reusable). Using a
+// mutable let + per-call wiring until #118 lands.
+export fn empty() stream = {
+	_empty_vt.reader = (&_empty_read): *reader;
+	_empty_vt.writer = (&_empty_write): *writer;
+	return &_empty_vt;
+};
+
 // errors — domain-agnostic error types. Mirrors ref/hare/errors/.
 //
 // Named-void tagged-union variants, so `(T | errors.invalid | ...)`
@@ -18079,6 +18116,16 @@ export fn sretretsize(c: *cgen, t: *node) i32 = {
 	// Primitives / aliased-to-primitives are never sret.
 	if (aliasprimsize(c, r.str) > 0) { return 0; };
 	if (streq(r.str, "str")) { return 0; };
+	// #129: same-module alias wins over any-module struct hit. Without
+	// this, `type stream = *vtable` (io) loses to memio.stream (56B
+	// struct) via structlookup's any-module fallback → spurious sret.
+	// Mirrors cstage cg_sret_retsize, which sees TY_PTR, not a name.
+	if (c != nil) {
+		let al: *node = aliassamemod(c, r.str);
+		if (al != nil) {
+			return sretretsize(c, al);
+		};
+	};
 	let si: *structinfo = structlookup(c, r.str);
 	if (si == nil) {
 		if (c != nil) {
@@ -18121,7 +18168,15 @@ export fn callsretsize(c: *cgen, n: *node) i32 = {
 	};
 	if (cn.len == 0) { return 0; };
 	let rtyp: *node = fnretlookupmod(c, cn, cmod);
-	return sretretsize(c, rtyp);
+	// #129: sretretsize must see the CALLEE's module context so
+	// aliassamemod resolves aliases from the callee's module (not the
+	// caller's). Mirrors cstage operating on resolved Type* objects
+	// (type_chase_named never has this confusion). Swap + restore.
+	let savedmod: str = c.curmod;
+	if (cmod.len > 0) { c.curmod = cmod; };
+	let r: i32 = sretretsize(c, rtyp);
+	c.curmod = savedmod;
+	return r;
 };
 
 fn structlookup(c: *cgen, name: str) *structinfo = {
@@ -31271,6 +31326,23 @@ fn cgassign(c: *cgen, n: *node) void = {
 											return;
 										};};
 									};
+									// #129: tagged-union field on global struct. LEAQ
+									// base(SB) into BX then the shared widener handles
+									// every rhs shape. Mirrors cstage cgen.c:4902
+									// is_global arm. Without this the generic TK_ASSIGN
+									// below truncates to 1 word, silently dropping tag
+									// and payload.
+									if (n.op == tkind.TK_ASSIGN
+									    && istaggedtype(c, fi.tnode)) {
+										let fsz: i32 = slotsize(c, fi.tnode);
+										emitline("\tLEAQ\t");
+										emitsymname(c, bn);
+										emitline("(SB), BX\n");
+										cgwidentaggedstore(c,
+										    fi.tnode.type_: *tinfo,
+										    n.rhs, "BX", fi.foff, fsz);
+										return;
+									};
 									if (n.op == tkind.TK_ASSIGN) {
 										cgexpr(c, n.rhs);
 										if (isstrtype(c, fi.tnode)) {

selfhost/cmd/wcc/cgenexpr.ww

@@ -9891,6 +9891,23 @@ fn cgassign(c: *cgen, n: *node) void = {
 											return;
 										};};
 									};
+									// #129: tagged-union field on global struct. LEAQ
+									// base(SB) into BX then the shared widener handles
+									// every rhs shape. Mirrors cstage cgen.c:4902
+									// is_global arm. Without this the generic TK_ASSIGN
+									// below truncates to 1 word, silently dropping tag
+									// and payload.
+									if (n.op == tkind.TK_ASSIGN
+									    && istaggedtype(c, fi.tnode)) {
+										let fsz: i32 = slotsize(c, fi.tnode);
+										emitline("\tLEAQ\t");
+										emitsymname(c, bn);
+										emitline("(SB), BX\n");
+										cgwidentaggedstore(c,
+										    fi.tnode.type_: *tinfo,
+										    n.rhs, "BX", fi.foff, fsz);
+										return;
+									};
 									if (n.op == tkind.TK_ASSIGN) {
 										cgexpr(c, n.rhs);
 										if (isstrtype(c, fi.tnode)) {

selfhost/cmd/wcc/cgenutil.ww

@@ -1880,6 +1880,16 @@ export fn sretretsize(c: *cgen, t: *node) i32 = {
 	// Primitives / aliased-to-primitives are never sret.
 	if (aliasprimsize(c, r.str) > 0) { return 0; };
 	if (streq(r.str, "str")) { return 0; };
+	// #129: same-module alias wins over any-module struct hit. Without
+	// this, `type stream = *vtable` (io) loses to memio.stream (56B
+	// struct) via structlookup's any-module fallback → spurious sret.
+	// Mirrors cstage cg_sret_retsize, which sees TY_PTR, not a name.
+	if (c != nil) {
+		let al: *node = aliassamemod(c, r.str);
+		if (al != nil) {
+			return sretretsize(c, al);
+		};
+	};
 	let si: *structinfo = structlookup(c, r.str);
 	if (si == nil) {
 		if (c != nil) {
@@ -1922,7 +1932,15 @@ export fn callsretsize(c: *cgen, n: *node) i32 = {
 	};
 	if (cn.len == 0) { return 0; };
 	let rtyp: *node = fnretlookupmod(c, cn, cmod);
-	return sretretsize(c, rtyp);
+	// #129: sretretsize must see the CALLEE's module context so
+	// aliassamemod resolves aliases from the callee's module (not the
+	// caller's). Mirrors cstage operating on resolved Type* objects
+	// (type_chase_named never has this confusion). Swap + restore.
+	let savedmod: str = c.curmod;
+	if (cmod.len > 0) { c.curmod = cmod; };
+	let r: i32 = sretretsize(c, rtyp);
+	c.curmod = savedmod;
+	return r;
 };
 
 fn structlookup(c: *cgen, name: str) *structinfo = {

selfhost/cmd/wwdump/main.combined.ww

@@ -15470,12 +15470,12 @@ export fn checkfile(c: *checker, file: *node) void = {
 // tagged unions instead of errno-style integer sentinels.
 //
 // This file owns the eof / underread variant tags; lib/io/stream.ww
-// owns the `vtable` + `stream` + read/write/close dispatchers, and
-// lib/io/types.ww owns the error union, mode/whence enums, and the
-// reader/writer/closer fn-type aliases. #94 fold-eFinal collapsed the
-// pre-vtable `stream` struct + `closed` tag into the single vtable
-// surface; the dispatchers are read/write/close (over `stream`),
-// final over `handle` at io fold-2 (#5).
+// owns the `vtable` + `stream` + read/write/close dispatchers and the
+// [[empty]] singleton, and lib/io/types.ww owns the error union,
+// mode/whence enums, and the reader/writer/closer fn-type aliases.
+// #94 fold-eFinal collapsed the pre-vtable `stream` struct + `closed`
+// tag into the single vtable surface; the dispatchers are read/write/
+// close (over `stream`), final over `handle` at io fold-2 (#5).
 package io;
 
 // eof — read past the end of the stream. Hare uses the `done`
@@ -15492,7 +15492,7 @@ export type underread = !i32;
 // stream — Hare-shaped vtable surface. Project #94 fold-eFinal.
 //
 // The single io stream surface (the fold-eFinal collapse retired the
-// pre-vtable `stream` struct + `closed` tag). Three exports:
+// pre-vtable `stream` struct + `closed` tag). Exports:
 //
 //   vtable     a struct of optional fn-pointer slots — reader/writer/
 //              closer per ref/hare/io/stream.ha:36-42. Hare spells the
@@ -15512,6 +15512,9 @@ export type underread = !i32;
 //              public `read`/`write`/`close`; ww has no `handle` yet
 //              (io fold-2, #5), so the dispatchers ARE the public
 //              surface and grow the `handle` match when #5 lands.
+//   empty      the discard+EOF stream (ref/hare/io/empty.ha:13). Lives
+//              here rather than io.ww so that 900_stdlib can compile
+//              io.ww standalone (io.ww has no cross-file type refs).
 //
 // Deferrals (drew-signed): `seeker` lands with io fold-2 (#5) once `off`
 // + `whence` plug into the signature. Hare's `?`-propagating `close`
@@ -15675,6 +15678,40 @@ export fn tell(h: handle) (off | error) = {
 	return seek(h, 0, whence.CUR);
 };
 
+// ---- empty stream -------------------------------------------------------
+//
+// ref/hare/io/empty.ha:4-17.
+//
+// Hare uses `const _empty_vt: vtable = { ... }` + `const empty: *stream`.
+// ww can't const-init a vtable struct with fn-ptr fields (#118), so the
+// vtable is a module-level `let` and [[empty]] is a function that wires
+// the fn-ptr slots on every call and returns the stream pointer.
+// Single-assignment on the same words: idempotent under re-entry.
+// Lives in stream.ww (not io.ww) so that 900_stdlib can compile io.ww
+// standalone without referencing the cross-file vtable/reader/writer types.
+
+fn _empty_read(s: stream, buf: []u8) (size | eof | error) = {
+	let e: eof;
+	return e;
+};
+
+fn _empty_write(s: stream, buf: []u8) (size | error) = {
+	return buf.len: size;
+};
+
+let _empty_vt: vtable;
+
+// empty — a stream that discards all writes (returning their size) and
+// returns EOF on every read. Mirrors ref/hare/io/empty.ha:13.
+// #118: const vtable init with fn-ptr fields is unwired (emit_struct_data
+// needs a two-pass reloc extension, node_fnptr_sym reusable). Using a
+// mutable let + per-call wiring until #118 lands.
+export fn empty() stream = {
+	_empty_vt.reader = (&_empty_read): *reader;
+	_empty_vt.writer = (&_empty_write): *writer;
+	return &_empty_vt;
+};
+
 // errors — domain-agnostic error types. Mirrors ref/hare/errors/.
 //
 // Named-void tagged-union variants, so `(T | errors.invalid | ...)`
@@ -18079,6 +18116,16 @@ export fn sretretsize(c: *cgen, t: *node) i32 = {
 	// Primitives / aliased-to-primitives are never sret.
 	if (aliasprimsize(c, r.str) > 0) { return 0; };
 	if (streq(r.str, "str")) { return 0; };
+	// #129: same-module alias wins over any-module struct hit. Without
+	// this, `type stream = *vtable` (io) loses to memio.stream (56B
+	// struct) via structlookup's any-module fallback → spurious sret.
+	// Mirrors cstage cg_sret_retsize, which sees TY_PTR, not a name.
+	if (c != nil) {
+		let al: *node = aliassamemod(c, r.str);
+		if (al != nil) {
+			return sretretsize(c, al);
+		};
+	};
 	let si: *structinfo = structlookup(c, r.str);
 	if (si == nil) {
 		if (c != nil) {
@@ -18121,7 +18168,15 @@ export fn callsretsize(c: *cgen, n: *node) i32 = {
 	};
 	if (cn.len == 0) { return 0; };
 	let rtyp: *node = fnretlookupmod(c, cn, cmod);
-	return sretretsize(c, rtyp);
+	// #129: sretretsize must see the CALLEE's module context so
+	// aliassamemod resolves aliases from the callee's module (not the
+	// caller's). Mirrors cstage operating on resolved Type* objects
+	// (type_chase_named never has this confusion). Swap + restore.
+	let savedmod: str = c.curmod;
+	if (cmod.len > 0) { c.curmod = cmod; };
+	let r: i32 = sretretsize(c, rtyp);
+	c.curmod = savedmod;
+	return r;
 };
 
 fn structlookup(c: *cgen, name: str) *structinfo = {
@@ -31271,6 +31326,23 @@ fn cgassign(c: *cgen, n: *node) void = {
 											return;
 										};};
 									};
+									// #129: tagged-union field on global struct. LEAQ
+									// base(SB) into BX then the shared widener handles
+									// every rhs shape. Mirrors cstage cgen.c:4902
+									// is_global arm. Without this the generic TK_ASSIGN
+									// below truncates to 1 word, silently dropping tag
+									// and payload.
+									if (n.op == tkind.TK_ASSIGN
+									    && istaggedtype(c, fi.tnode)) {
+										let fsz: i32 = slotsize(c, fi.tnode);
+										emitline("\tLEAQ\t");
+										emitsymname(c, bn);
+										emitline("(SB), BX\n");
+										cgwidentaggedstore(c,
+										    fi.tnode.type_: *tinfo,
+										    n.rhs, "BX", fi.foff, fsz);
+										return;
+									};
 									if (n.op == tkind.TK_ASSIGN) {
 										cgexpr(c, n.rhs);
 										if (isstrtype(c, fi.tnode)) {

test/wcc/989_lib_byteid.c

@@ -95,6 +95,15 @@ static const struct ent ents[] = {
 	{ .fixture = "lib/strings/stringstest.ww",         .mode = M_ID },
 	{ .fixture = "lib/temp/temptest.ww",               .mode = M_ID },
 	{ .fixture = "lib/time/timetest.ww",               .mode = M_ID },
+	/* graduated from #59.2 DIVERGE by the #129 sretretsize fixes
+	 * (same-module alias priority + callsretsize callee-context swap;
+	 * bufio imports io so cross-module io.stream returns were spuriously
+	 * sret-classified, producing divergent caller prologues) */
+	{ .fixture = "lib/bufio/bufiotest.ww",             .mode = M_ID },
+	/* graduated from #59.6 DIVERGE by the #129 same fix (fmt imports
+	 * io transitively through bufio; sretretsize curmod swap closed
+	 * the getopt.printusage spurious LEAQ DI caller divergence) */
+	{ .fixture = "lib/fmt/fmttest.ww",                 .mode = M_ID },
 	/* fixtureless modules, import-probe shape */
 	{ .probe = "package main;\nimport sort;\nfn main() i32 = { return 0; };\n",
 	  .mode = M_ID, .sentinel = "package sort;", .moddir = "lib/sort" },
@@ -119,15 +128,13 @@ static const struct ent ents[] = {
 	/* -------- documented-allowed cs≠ww (task #59) ------------ */
 	{ .fixture = "lib/ascii/asciitest.ww",
 	  .mode = M_DIVERGE, .cite = "#59.1" },
-	{ .fixture = "lib/bufio/bufiotest.ww",
-	  .mode = M_DIVERGE, .cite = "#59.2" },
+	/* #59.2 bufio graduated to M_ID above (#129 fix) */
 	{ .fixture = "lib/encoding/base64/base64_test.ww",
 	  .mode = M_DIVERGE, .cite = "#59.3" },
 	/* #59.4 hextest graduated to M_ID above (#22a reviewer fixes) */
 	{ .fixture = "lib/errors/errnotest.ww",
 	  .mode = M_DIVERGE, .cite = "#59.5" },
-	{ .fixture = "lib/fmt/fmttest.ww",
-	  .mode = M_DIVERGE, .cite = "#59.6" },
+	/* #59.6 fmt graduated to M_ID above (#129 fix) */
 	/* #59.7 siphash graduated to M_ID above (#61 fix) */
 	{ .fixture = "lib/log/logtest.ww",
 	  .mode = M_DIVERGE, .cite = "#59.8" },