cgen, x.json2: fix anon struct encode (fix #24836) (#24852)

vlib/v/gen/c/fn.v

@@ -2767,7 +2767,8 @@ fn (mut g Gen) ref_or_deref_arg(arg ast.CallArg, expected_type ast.Type, lang as
 		g.expr_with_cast(arg.expr, arg_typ, expected_type)
 		g.write('.data')
 		return
-	} else if arg.expr is ast.Ident && arg_sym.info is ast.Struct && arg_sym.info.is_anon {
+	} else if arg.expr is ast.Ident && arg_sym.info is ast.Struct && arg_sym.info.is_anon
+		&& !expected_type.has_flag(.generic) {
 		// make anon struct struct compatible with another anon struct declaration
 		g.write('*(${g.cc_type(expected_type, false)}*)&')
 	}

vlib/x/json2/tests/encode_anon_struct_test.v

@@ -0,0 +1,24 @@
+import x.json2
+
+struct Disk {
+	dev  string
+	size ?struct {
+		value u64
+	}
+}
+
+fn test_main() {
+	disk := Disk{
+		size: struct {
+			value: 123
+		}
+	}
+	disk_str := json2.encode[Disk](disk)
+	assert disk_str == '{"dev":"","size":{"value":123}}'
+}
+
+fn test_none() {
+	disk := Disk{}
+	disk_str := json2.encode[Disk](disk)
+	assert disk_str == '{"dev":""}'
+}