From 5d7958db7d979de19102a22e2acaf52efb7eb599 Mon Sep 17 00:00:00 2001
From: Larsimusrex <lars.luis@gmail.com>
Date: Sat, 13 Sep 2025 06:57:01 +0200
Subject: [PATCH] cgen: fix generic cast to sumtype of empty struct (fix
 #25263) (#25290)

---
 vlib/v/gen/c/cgen.v                            |  3 ++-
 .../tests/sumtypes/sumtype_empty_struct_test.v | 18 ++++++++++++++++++
 vlib/x/json2/types.v                           |  4 +---
 3 files changed, 21 insertions(+), 4 deletions(-)
 create mode 100644 vlib/v/tests/sumtypes/sumtype_empty_struct_test.v

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index d7675eb2b..db2fa815b 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -7574,8 +7574,9 @@ fn (mut g Gen) type_default_impl(typ_ ast.Type, decode_sumtype bool) string {
 			}
 		}
 		.struct {
-			mut has_none_zero := false
 			info := sym.info as ast.Struct
+			mut has_none_zero := info.fields.len == 0
+
 			mut init_str := if info.is_anon && !g.inside_global_decl {
 				'(${g.styp(typ)}){'
 			} else {
diff --git a/vlib/v/tests/sumtypes/sumtype_empty_struct_test.v b/vlib/v/tests/sumtypes/sumtype_empty_struct_test.v
new file mode 100644
index 000000000..40bd3402a
--- /dev/null
+++ b/vlib/v/tests/sumtypes/sumtype_empty_struct_test.v
@@ -0,0 +1,18 @@
+struct Empty {
+}
+
+type Sum = int | Empty
+
+fn do_generic[T](val T) T {
+	$for v in Sum.variants {
+		if val is v {
+			return T(v)
+		}
+	}
+	return T{}
+}
+
+fn test_empty_cast() {
+	assert do_generic(Sum(0)) == Sum(0)
+	assert do_generic(Sum(Empty{})) == Sum(Empty{})
+}
diff --git a/vlib/x/json2/types.v b/vlib/x/json2/types.v
index b9a71fd5b..e60068670 100644
--- a/vlib/x/json2/types.v
+++ b/vlib/x/json2/types.v
@@ -24,9 +24,7 @@ pub type Any = []Any
 	| Null
 
 // Null is a simple representation of the `null` value in JSON.
-pub struct Null {
-	is_null bool = true
-}
+pub struct Null {}
 
 // null is an instance of the Null type, to ease comparisons with it.
 pub const null = Null{}
-- 
2.39.5