checker, cgen: allow $for in alias type (#16810)

vlib/v/checker/comptime.v

@@ -171,7 +171,7 @@ fn (mut c Checker) comptime_selector(mut node ast.ComptimeSelector) ast.Type {
 
 fn (mut c Checker) comptime_for(node ast.ComptimeFor) {
 	typ := c.unwrap_generic(node.typ)
-	sym := c.table.sym(typ)
+	sym := c.table.final_sym(typ)
 	if sym.kind == .placeholder || typ.has_flag(.generic) {
 		c.error('unknown type `${sym.name}`', node.typ_pos)
 	}

vlib/v/gen/c/comptime.v

@@ -507,7 +507,7 @@ fn (mut g Gen) comptime_if_cond(cond ast.Expr, pkg_exist bool) bool {
 }
 
 fn (mut g Gen) comptime_for(node ast.ComptimeFor) {
-	sym := g.table.sym(g.unwrap_generic(node.typ))
+	sym := g.table.final_sym(g.unwrap_generic(node.typ))
 	g.writeln('/* \$for ${node.val_var} in ${sym.name}(${node.kind.str()}) */ {')
 	g.indent++
 	// vweb_result_type := ast.new_type(g.table.find_type_idx('vweb.Result'))

vlib/v/tests/forcomp_alias_type_test.v

@@ -0,0 +1,25 @@
+module main
+
+struct Aa {
+	sub AliasType
+}
+
+type AliasType = Bb
+
+struct Bb {
+	a int
+}
+
+fn encode_struct[U](val U, mut out []string) []string {
+	$for field in U.fields {
+		encode_struct(val.$(field.name), mut out)
+		out << field.str()
+	}
+	return out
+}
+
+fn test_main() {
+	aa := Aa{}
+	mut out := []string{}
+	assert encode_struct(aa, mut out).len == 2
+}