checker: disallow none type in struct fields (#16425)

vlib/v/checker/struct.v

@@ -84,6 +84,10 @@ pub fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 				c.error('cannot use multi return as field type', field.type_pos)
 			}
 
+			if sym.kind == .none_ {
+				c.error('cannot use `none` as field type', field.type_pos)
+			}
+
 			if field.has_default_expr {
 				c.expected_type = field.typ
 				default_expr_type := c.expr(field.default_expr)

vlib/v/checker/tests/struct_none_field_type_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/struct_none_field_type_err.vv:2:7: error: cannot use `none` as field type
+    1 | struct Jaca {
+    2 |     lala none
+      |          ~~~~
+    3 | }
+    4 |

vlib/v/checker/tests/struct_none_field_type_err.vv

@@ -0,0 +1,12 @@
+struct Jaca {
+	lala none
+}
+
+fn f() {
+	mut s := Jaca{
+		lala: none
+	}
+	println(s)
+}
+
+f()