cgen: improve uninitialized option struct field declaration (#20129)

vlib/v/gen/c/struct.v

@@ -378,8 +378,7 @@ fn (mut g Gen) zero_struct_field(field ast.StructField) bool {
 		}
 
 		if field.default_expr is ast.None {
-			tmp_var := g.new_tmp_var()
-			g.expr_with_tmp_var(ast.None{}, ast.none_type, field.typ, tmp_var)
+			g.gen_option_error(field.typ, ast.None{})
 			return true
 		} else if field.typ.has_flag(.option) {
 			tmp_var := g.new_tmp_var()
@@ -394,8 +393,7 @@ fn (mut g Gen) zero_struct_field(field ast.StructField) bool {
 		}
 		g.expr(field.default_expr)
 	} else if field.typ.has_flag(.option) {
-		tmp_var := g.new_tmp_var()
-		g.expr_with_tmp_var(ast.None{}, ast.none_type, field.typ, tmp_var)
+		g.gen_option_error(field.typ, ast.None{})
 		return true
 	} else if sym.info is ast.ArrayFixed {
 		g.write('{')

vlib/v/tests/option_struct_field_init_test.v

@@ -0,0 +1,34 @@
+pub struct Client {
+pub:
+	token   string
+	intents int
+mut:
+	ready    bool
+	sequence ?int
+}
+
+enum Intents {
+	foo
+}
+
+pub type IntentsOrInt = Intents | int
+
+@[params]
+pub struct BotConfig {
+pub:
+	intents IntentsOrInt
+}
+
+pub fn bot(token string, config BotConfig) Client {
+	return Client{
+		token: 'Bot ${token}'
+		intents: match config.intents {
+			Intents { int(config.intents) }
+			int { config.intents }
+		}
+	}
+}
+
+fn test_main() {
+	assert true
+}