checker: disallow expr is Type if expr is Optional

vlib/v/checker/infix.v

@@ -781,6 +781,10 @@ fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 			if typ != ast.no_type {
 				typ_sym := c.table.sym(typ)
 				op := node.op.str()
+				if left_type.has_flag(.option) {
+					c.error('${node.left} is an Optional, it needs to be unwrapped first',
+						node.left.pos())
+				}
 				if typ_sym.kind == .placeholder {
 					c.error('${op}: type `${typ_sym.name}` does not exist', right_expr.pos())
 				}

vlib/v/checker/tests/opt_is_op_check_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/opt_is_op_check_err.vv:13:7: error: t.a is an Optional, it needs to be unwrapped first
+   11 |         a: s
+   12 |     }
+   13 |     if t.a is string {
+      |          ^
+   14 |         r(t.a)
+   15 |     }

vlib/v/checker/tests/opt_is_op_check_err.vv

@@ -0,0 +1,21 @@
+type Foo = int | string
+
+struct Struct {
+	a ?Foo
+}
+
+fn r(a string) {}
+
+fn t(s ?Foo) {
+	mut t := Struct{
+		a: s
+	}
+	if t.a is string {
+		r(t.a)
+	}
+}
+
+fn main() {
+	s := ?Foo('hello')
+	t(s)
+}

vlib/v/checker/tests/orm_op_with_option_and_none.out

@@ -47,3 +47,17 @@ vlib/v/checker/tests/orm_op_with_option_and_none.vv:45:30: warning: comparison w
       |                                    ~~
    46 |     }!
    47 |
+vlib/v/checker/tests/orm_op_with_option_and_none.vv:49:25: error: name is an Optional, it needs to be unwrapped first
+   47 | 
+   48 |     _ := sql db {
+   49 |         select from Foo where name is none
+      |                               ~~~~
+   50 |     }!
+   51 |
+vlib/v/checker/tests/orm_op_with_option_and_none.vv:53:25: error: name is an Optional, it needs to be unwrapped first
+   51 | 
+   52 |     _ := sql db {
+   53 |         select from Foo where name !is none
+      |                               ~~~~
+   54 |     }!
+   55 | }