checker: handle unwrapped optionals in infix exprs

vlib/v/checker/check_types.v

@@ -16,6 +16,11 @@ pub fn (mut c Checker) check_basic(got table.Type, expected table.Type) bool {
 	exp_idx := t.unalias_num_type(expected).idx()
 	// got_is_ptr := got.is_ptr()
 	exp_is_ptr := expected.is_ptr()
+	// exp_is_optional := expected.has_flag(.optional)
+	// got_is_optional := got.has_flag(.optional)
+	// if (exp_is_optional && !got_is_optional) || (!exp_is_optional && got_is_optional) {
+	// return false
+	//}
 	// println('check: $got_type_sym.name, $exp_type_sym.name')
 	// # NOTE: use idxs here, and symbols below for perf
 	if got_idx == exp_idx {

vlib/v/checker/checker.v

@@ -910,6 +910,12 @@ pub fn (mut c Checker) infix_expr(mut infix_expr ast.InfixExpr) table.Type {
 	} else if c.table.type_kind(right_type) == .sum_type {
 		c.error('cannot use operator `$infix_expr.op` with `$right.name`', infix_expr.pos)
 	}
+	// TODO move this to symmetric_check? Right now it would break `return 0` for `fn()?int `
+	left_is_optional := left_type.has_flag(.optional)
+	right_is_optional := right_type.has_flag(.optional)
+	if (left_is_optional && !right_is_optional) || (!left_is_optional && right_is_optional) {
+		c.error('unwrapped optional cannot be used in an infix expression', infix_expr.pos)
+	}
 	// Dual sides check (compatibility check)
 	if !c.symmetric_check(right_type, left_type) && !c.pref.translated {
 		// for type-unresolved consts

vlib/v/checker/tests/unwrapped_optional_infix.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/unwrapped_optional_infix.vv:5:16: error: unwrapped optional cannot be used in an infix expression
+    3 | }
+    4 |
+    5 | println(test() == "")
+      |

vlib/v/checker/tests/unwrapped_optional_infix.vv

@@ -0,0 +1,5 @@
+fn test() ?string {
+	return ""
+}
+
+println(test() == "")