cgen: fix none initialization to struct member (#18295)

vlib/v/checker/struct.v

@@ -163,6 +163,10 @@ fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 								field.default_expr.pos)
 						}
 					}
+					if field.typ.has_flag(.option) && field.default_expr is ast.None {
+						c.warn('unnecessary default value of `none`: struct fields are zeroed by default',
+							field.default_expr.pos)
+					}
 					continue
 				}
 				if field.typ in ast.unsigned_integer_type_idxs {

vlib/v/gen/c/struct.v

@@ -346,7 +346,11 @@ fn (mut g Gen) zero_struct_field(field ast.StructField) bool {
 			return true
 		}
 
-		if field.typ.has_flag(.option) {
+		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)
+			return true
+		} else if field.typ.has_flag(.option) {
 			tmp_var := g.new_tmp_var()
 			g.expr_with_tmp_var(field.default_expr, field.default_expr_typ, field.typ,
 				tmp_var)

vlib/v/tests/generic_struct_recursive_test.v

@@ -0,0 +1,20 @@
+struct Node[T] {
+mut:
+	value T
+	next  ?&Node[T] = none
+}
+
+fn test_main() {
+	mut n := Node[int]{
+		value: 1
+	}
+	mut m := Node[int]{
+		value: 2
+	}
+	n.next = &m
+	a := n.next or { return }
+	dump(a)
+	dump(m)
+	dump(n)
+	assert a.value == 2
+}