cgen: default the hi bound of a slice/str-field slice, both stages

x.slicefield[:] / x.strfield[2:] emitted MOVQ $0 for the omitted hi
bound on BOTH stages (N_IDENT-gated dispatch; symmetric, so byte
identity never caught it) in all four sites: cgexpr N_SLICE + pushargs
(cstage), cgslice + pushargsrev (wwstage). The new arm re-evaluates
the pure field read for its {ptr,len,cap} header and takes .len,
covering local, viaptr, dot-chain, (*p), arr[i], and global inners.
Call inners still loud-reject upstream. Sibling of the #252/#257
array-field arms.

cmd/w6c/cgen.c

@@ -10112,6 +10112,14 @@ cgexpr(Cg *c, Node *n, Local *locals)
 					} else {
 						ins2(c, A_MOVQ, amem(D_BP, boff + 8), areg(D_AX));
 					}
+				} else if (base->kind == N_DOT && bu &&
+				    (bu->kind == TY_SLICE || bu->kind == TY_STR)) {
+					/* slice/str FIELD base — the arg twin
+					 * of the cgexpr N_SLICE default-hi
+					 * arm above; same re-eval-for-header
+					 * rationale. */
+					cgexpr(c, base, locals);
+					ins2(c, A_MOVQ, areg(D_BX), areg(D_AX));
 				} else {
 					cgexpr_int(c, 0);
 				}
@@ -13242,6 +13250,18 @@ cgexpr(Cg *c, Node *n, Local *locals)
 				ins2(c, A_MOVQ, amem(D_BP, boff + 8),
 				    areg(D_AX));
 			}
+		} else if (base && base->kind == N_DOT && bu &&
+		    (bu->kind == TY_SLICE || bu->kind == TY_STR)) {
+			/* slice/str FIELD base (#252's slice twin): the old
+			 * fall-through emitted $0 — a silent 0/negative len on
+			 * BOTH stages, byteid-blind. Re-evaluate the field
+			 * read for its header (a pure place: call inners
+			 * loud-reject upstream; cgexpr's N_DOT slice/str arms
+			 * leave AX=ptr, BX=len, CX=cap for every supported
+			 * inner — ident, chain, (*p), arr[i]) and take .len.
+			 * All scratch regs are dead at the hi step. */
+			cgexpr(c, base, locals);
+			ins2(c, A_MOVQ, areg(D_BX), areg(D_AX));
 		} else {
 			cgexpr_int(c, 0);
 		}

internal/wwfixture/types.ww

@@ -1,13 +1,13 @@
 package wwfixture;
 
 def protocolversion: i32 = 1;
-def corpuscount: i32 = 1230;
+def corpuscount: i32 = 1233;
 def errorcount: i32 = 314;
 def compilecount: i32 = 12;
-def runcount: i32 = 138;
+def runcount: i32 = 141;
 def runexitcount: i32 = 766;
-def nativecount: i32 = 2460;
-def corpushash: str = "fd04059b05a19ae63f1bec3edaefedf1d8ce9a505302a153434457d0fb9b9eca";
+def nativecount: i32 = 2466;
+def corpushash: str = "65d3642dfb77b486f5aed23044a979bbce37c36fbbcb7a7aedae20d87c7b0e4f";
 
 type directive = enum i32 {
 	ERROR = 0,

selfhost/cmd/wcc/cgenexpr.ww

@@ -3000,6 +3000,17 @@ fn cgslice(c: *cgen, n: *syntax.node) void = {
 		emitline("\tMOVQ\t$");
 		emitint(dotbu.alen: i64);
 		emitline(", AX\n");
+	} else { if (dotbu != nil && (dotbu.kind == syntax.tykind.TY_SLICE
+	    || dotbu.kind == syntax.tykind.TY_STR)) {
+		// slice/str FIELD base (#252's slice twin): the old
+		// fall-through emitted $0 — silent 0/negative len on BOTH
+		// stages, byteid-blind. Re-evaluate the field read for its
+		// header (pure place: call inners loud-reject upstream;
+		// cgdot's slice/str arms leave AX=ptr, BX=len, CX=cap for
+		// every supported inner) and take .len. Scratch regs are
+		// dead at the hi step. Cstage twin: cgen.c N_SLICE.
+		cgexpr(c, base);
+		emitline("\tMOVQ\tBX, AX\n");
 	} else { if (arrlittn != nil) {
 		// #31: default-hi for the arrlit base = its element count (the
 		// stashed [count]T tnode's .rhs intlit).
@@ -3014,7 +3025,7 @@ fn cgslice(c: *cgen, n: *syntax.node) void = {
 		emitline(", AX\n");
 	} else {
 		emitline("\tMOVQ\t$0, AX\n");
-	};};};};};
+	};};};};};};
 	emitline("\tMOVQ\tAX, BX\n");
 	emitline("\tPOPQ\tCX\n");
 	emitline("\tPOPQ\tAX\n");

selfhost/cmd/wcc/cgenutil.ww

@@ -782,9 +782,16 @@ fn pushargsrev(c: *cgen, arg: *syntax.node, param: *syntax.node, memphase: bool,
 			emitline("\tMOVQ\t$");
 			emitint(dotbu.alen: i64);
 			emitline(", AX\n");
+		} else { if (dotbu != nil && (dotbu.kind == syntax.tykind.TY_SLICE
+		    || dotbu.kind == syntax.tykind.TY_STR)) {
+			// slice/str FIELD base — the arg twin of the cgslice
+			// default-hi arm; same re-eval-for-header rationale.
+			// Cstage twin: cgen.c pushargs N_SLICE.
+			cgexpr(c, base);
+			emitline("\tMOVQ\tBX, AX\n");
 		} else {
 			emitline("\tMOVQ\t$0, AX\n");
-		};};};};
+		};};};};};
 		emitline("\tPUSHQ\tAX\n");
 		// lo (default 0) → AX
 		if (lo != nil) { cgexpr(c, lo); }

test/wcc/data/dotfield_slice_defhi/case.ww

@@ -0,0 +1,49 @@
+//ww:run
+// Default-hi slice of a slice/str FIELD (`x.f[:]`, `x.f[2:]`) —
+// expression position. The pre-fix default-hi dispatch was
+// N_IDENT-gated on BOTH stages, so an N_DOT base fell to MOVQ $0
+// (len 0 / negative, byteid-blind). Covers local, viaptr, dot-chain,
+// (*p) spelling, and struct-array-element inners.
+package main;
+
+type inner = struct {
+	buf: []i32,
+	name: str,
+};
+
+type outer = struct {
+	pad: i64,
+	in: inner,
+};
+
+export fn main() int = {
+	let arr: [4]i32 = [10, 20, 30, 40];
+	let v: inner = inner{ buf = arr[:], name = "hello" };
+	let b1: []i32 = v.buf[:];
+	if (len(b1) != 4) { return 1; };
+	let b2: []i32 = v.buf[1:];
+	if (len(b2) != 3) { return 2; };
+	if (b2[0] != 20) { return 3; };
+	let s1: str = v.name[2:];
+	if (len(s1) != 3) { return 4; };
+	let p: *inner = &v;
+	let b3: []i32 = p.buf[:];
+	if (len(b3) != 4) { return 5; };
+	let s2: str = p.name[1:];
+	if (len(s2) != 4) { return 6; };
+	let o: outer = outer{ pad = 7, in = inner{ buf = arr[:], name = "world" } };
+	let b4: []i32 = o.in.buf[:];
+	if (len(b4) != 4) { return 7; };
+	let b5: []i32 = o.in.buf[2:];
+	if (len(b5) != 2) { return 8; };
+	if (b5[1] != 40) { return 9; };
+	let s3: str = o.in.name[3:];
+	if (len(s3) != 2) { return 10; };
+	let b6: []i32 = (*p).buf[:];
+	if (len(b6) != 4) { return 11; };
+	let xs: [2]inner = [v, v];
+	let b7: []i32 = xs[1].buf[:];
+	if (len(b7) != 4) { return 12; };
+	if (b7[3] != 40) { return 13; };
+	return 0;
+};

test/wcc/data/dotfield_slice_defhi_arg/case.ww

@@ -0,0 +1,46 @@
+//ww:run
+// Default-hi slice of a slice/str FIELD in call-ARG position — the
+// pushargs twin of dotfield_slice_defhi. Pre-fix the pushargs
+// N_SLICE default-hi arm was N_IDENT-gated on BOTH stages, so
+// callees received a 0-length slice.
+package main;
+
+type inner = struct {
+	buf: []i32,
+	name: str,
+};
+
+type outer = struct {
+	pad: i64,
+	in: inner,
+};
+
+fn sum(xs: []i32) i32 = {
+	let t: i32 = 0;
+	let i: i32 = 0;
+	for (i < len(xs)) {
+		t += xs[i];
+		i += 1;
+	};
+	return t;
+};
+
+fn slen(s: str) i32 = {
+	return len(s);
+};
+
+export fn main() int = {
+	let arr: [4]i32 = [10, 20, 30, 40];
+	let v: inner = inner{ buf = arr[:], name = "hello" };
+	if (sum(v.buf[:]) != 100) { return 1; };
+	if (sum(v.buf[2:]) != 70) { return 2; };
+	if (slen(v.name[1:]) != 4) { return 3; };
+	let p: *inner = &v;
+	if (sum(p.buf[:]) != 100) { return 4; };
+	if (slen(p.name[2:]) != 3) { return 5; };
+	let o: outer = outer{ pad = 7, in = inner{ buf = arr[:], name = "world" } };
+	if (sum(o.in.buf[:]) != 100) { return 6; };
+	if (sum(o.in.buf[1:]) != 90) { return 7; };
+	if (slen(o.in.name[4:]) != 1) { return 8; };
+	return 0;
+};

test/wcc/data/dotfield_slice_defhi_global/case.ww

@@ -0,0 +1,51 @@
+//ww:run
+// Default-hi slice of a slice/str FIELD on module-GLOBAL struct
+// bases (single dot and chain), expression + arg positions — the
+// global leg of dotfield_slice_defhi.
+package main;
+
+type inner = struct {
+	buf: []i32,
+	name: str,
+};
+
+type outer = struct {
+	pad: i64,
+	in: inner,
+};
+
+let g: inner = inner{};
+let go: outer = outer{};
+
+fn sum(xs: []i32) i32 = {
+	let t: i32 = 0;
+	let i: i32 = 0;
+	for (i < len(xs)) {
+		t += xs[i];
+		i += 1;
+	};
+	return t;
+};
+
+export fn main() int = {
+	let arr: [4]i32 = [10, 20, 30, 40];
+	g.buf = arr[:];
+	g.name = "globals";
+	let b1: []i32 = g.buf[:];
+	if (len(b1) != 4) { return 1; };
+	let b2: []i32 = g.buf[2:];
+	if (len(b2) != 2) { return 2; };
+	if (b2[0] != 30) { return 3; };
+	let s1: str = g.name[3:];
+	if (len(s1) != 4) { return 4; };
+	if (sum(g.buf[:]) != 100) { return 5; };
+	if (sum(g.buf[1:]) != 90) { return 6; };
+	go.in.buf = arr[:];
+	go.in.name = "chain";
+	let b3: []i32 = go.in.buf[:];
+	if (len(b3) != 4) { return 7; };
+	if (sum(go.in.buf[2:]) != 70) { return 8; };
+	let s2: str = go.in.name[1:];
+	if (len(s2) != 4) { return 9; };
+	return 0;
+};