wcc: route is/as variant lookup through tinfo.params (#198)

checkisas walked the unflattened AST u.list via casevariantin (typeeqast
streq), so any variant introduced via a `...inner` spread was invisible
and rejected as "is/as: not a variant of operand". Repro:

    type rsh = (size | io.eof | ...io.error);
    let r: rsh = 42: size;
    if (r is io.underread) ...    -- pre-fix wwstage REJECTS

io.underread is in io.error.params, which tinfofornode splices into the
parent at L1827-1836, but the AST u.list still holds the single
`...io.error` entry that streq("io.underread", "io.error") rejects.

Route through flatvariantidxt — the same Phase-N helper #179 cgmatch
and #66 cgtagvariantidx already key off. Mirrors cstage cmd/wcc/check.c
:1662-1675 u->params + type_eq. Falls back to casevariantin AST walk
when tinfo isn't available (defensive — non-#198 path stays as-is).

project_tinfo_lossy_nominal: name-keying was the pre-Phase-N workaround
for tinfo lossy on nominal identity; typeeq inside flatvariantidxt now
handles NAMED ptr-id (#64), so the checker pair aligns with cgen on the
flattened-variant axis.

Closes the cgen-drain mini-cluster (#201 -> #199 -> #200 -> #198).

773_isas_spread_variant: 5 rows (spread_is_inline_variant,
direct_cross_mod_tagged, cross_mod_named_void, same_module_variant,
spread_as_inline_payload). Rows 2-4 byte-id; rows 1/5 skip byte-id due
to layout-asymmetry on `...wrapper` (cstage flattens at resolve_type,
wwstage computes maxsz off vt.size of the un-spliced alias) — sibling
not blocking the checker correctness fix.

selfhost/cmd/w6c/main.combined.ww

@@ -13470,16 +13470,29 @@ fn checkisas(c: *checker, n: *node) void = {
 	};
 	let want: *node = n.rhs;
 	if (want == nil) { return; };
-	if (!casevariantin(u, want)) {
-		os.write(2, "is/as: not a variant of operand".ptr, 31u64);
-		if (want.kind == nkind.N_TNAME) {
-			os.write(2, " (".ptr, 2u64);
-			os.write(2, want.str.ptr, want.str.len: u64);
-			os.write(2, ")".ptr, 1u64);
-		};
-		os.write(2, "\n".ptr, 1u64);
-		c.errs += 1;
+	// #198: route through tinfo.params (the #61a-flattened chain built
+	// at L1789-1845 N_TTAGGED) — the prior AST u.list walk via
+	// casevariantin false-rejects every variant that arrives via a
+	// `...inner` spread. Mirrors cstage cmd/wcc/check.c:1662-1675
+	// u->params + type_eq, and matches cgen's own #179 cgmatch / #66
+	// Phase-N flatvariantidxt lookup (the SSoT cgtagvariantidx already
+	// keys off at cgenexpr.ww:155). project_tinfo_lossy_nominal: name-
+	// keying was the pre-Phase-N workaround for tinfo lossy on nominal
+	// identity; typeeq inside flatvariantidxt now handles NAMED ptr-id.
+	let utinfo: *tinfo = tinfofornode(c, u);
+	let wanttinfo: *tinfo = tinfofornode(c, want);
+	if (utinfo != nil) { if (wanttinfo != nil) {
+		if (flatvariantidxt(utinfo, wanttinfo) >= 0) { return; };
+	}; };
+	if (casevariantin(u, want)) { return; };
+	os.write(2, "is/as: not a variant of operand".ptr, 31u64);
+	if (want.kind == nkind.N_TNAME) {
+		os.write(2, " (".ptr, 2u64);
+		os.write(2, want.str.ptr, want.str.len: u64);
+		os.write(2, ")".ptr, 1u64);
 	};
+	os.write(2, "\n".ptr, 1u64);
+	c.errs += 1;
 };
 
 // ---- ? subset propagation --------------------------------------------

selfhost/cmd/wcc/check.ww

@@ -3440,16 +3440,29 @@ fn checkisas(c: *checker, n: *node) void = {
 	};
 	let want: *node = n.rhs;
 	if (want == nil) { return; };
-	if (!casevariantin(u, want)) {
-		os.write(2, "is/as: not a variant of operand".ptr, 31u64);
-		if (want.kind == nkind.N_TNAME) {
-			os.write(2, " (".ptr, 2u64);
-			os.write(2, want.str.ptr, want.str.len: u64);
-			os.write(2, ")".ptr, 1u64);
-		};
-		os.write(2, "\n".ptr, 1u64);
-		c.errs += 1;
+	// #198: route through tinfo.params (the #61a-flattened chain built
+	// at L1789-1845 N_TTAGGED) — the prior AST u.list walk via
+	// casevariantin false-rejects every variant that arrives via a
+	// `...inner` spread. Mirrors cstage cmd/wcc/check.c:1662-1675
+	// u->params + type_eq, and matches cgen's own #179 cgmatch / #66
+	// Phase-N flatvariantidxt lookup (the SSoT cgtagvariantidx already
+	// keys off at cgenexpr.ww:155). project_tinfo_lossy_nominal: name-
+	// keying was the pre-Phase-N workaround for tinfo lossy on nominal
+	// identity; typeeq inside flatvariantidxt now handles NAMED ptr-id.
+	let utinfo: *tinfo = tinfofornode(c, u);
+	let wanttinfo: *tinfo = tinfofornode(c, want);
+	if (utinfo != nil) { if (wanttinfo != nil) {
+		if (flatvariantidxt(utinfo, wanttinfo) >= 0) { return; };
+	}; };
+	if (casevariantin(u, want)) { return; };
+	os.write(2, "is/as: not a variant of operand".ptr, 31u64);
+	if (want.kind == nkind.N_TNAME) {
+		os.write(2, " (".ptr, 2u64);
+		os.write(2, want.str.ptr, want.str.len: u64);
+		os.write(2, ")".ptr, 1u64);
 	};
+	os.write(2, "\n".ptr, 1u64);
+	c.errs += 1;
 };
 
 // ---- ? subset propagation --------------------------------------------

selfhost/cmd/wwdump/main.combined.ww

@@ -13470,16 +13470,29 @@ fn checkisas(c: *checker, n: *node) void = {
 	};
 	let want: *node = n.rhs;
 	if (want == nil) { return; };
-	if (!casevariantin(u, want)) {
-		os.write(2, "is/as: not a variant of operand".ptr, 31u64);
-		if (want.kind == nkind.N_TNAME) {
-			os.write(2, " (".ptr, 2u64);
-			os.write(2, want.str.ptr, want.str.len: u64);
-			os.write(2, ")".ptr, 1u64);
-		};
-		os.write(2, "\n".ptr, 1u64);
-		c.errs += 1;
+	// #198: route through tinfo.params (the #61a-flattened chain built
+	// at L1789-1845 N_TTAGGED) — the prior AST u.list walk via
+	// casevariantin false-rejects every variant that arrives via a
+	// `...inner` spread. Mirrors cstage cmd/wcc/check.c:1662-1675
+	// u->params + type_eq, and matches cgen's own #179 cgmatch / #66
+	// Phase-N flatvariantidxt lookup (the SSoT cgtagvariantidx already
+	// keys off at cgenexpr.ww:155). project_tinfo_lossy_nominal: name-
+	// keying was the pre-Phase-N workaround for tinfo lossy on nominal
+	// identity; typeeq inside flatvariantidxt now handles NAMED ptr-id.
+	let utinfo: *tinfo = tinfofornode(c, u);
+	let wanttinfo: *tinfo = tinfofornode(c, want);
+	if (utinfo != nil) { if (wanttinfo != nil) {
+		if (flatvariantidxt(utinfo, wanttinfo) >= 0) { return; };
+	}; };
+	if (casevariantin(u, want)) { return; };
+	os.write(2, "is/as: not a variant of operand".ptr, 31u64);
+	if (want.kind == nkind.N_TNAME) {
+		os.write(2, " (".ptr, 2u64);
+		os.write(2, want.str.ptr, want.str.len: u64);
+		os.write(2, ")".ptr, 1u64);
 	};
+	os.write(2, "\n".ptr, 1u64);
+	c.errs += 1;
 };
 
 // ---- ? subset propagation --------------------------------------------