checker: add check preventing `if x:=non_optional() {}`

vlib/v/checker/checker.v

@@ -2809,6 +2809,9 @@ pub fn (mut c Checker) expr(node ast.Expr) table.Type {
 		}
 		ast.IfGuardExpr {
 			node.expr_type = c.expr(node.expr)
+			if !node.expr_type.has_flag(.optional) {
+				c.error('expression should return an option', node.expr.position())
+			}
 			return table.bool_type
 		}
 		ast.IndexExpr {

vlib/v/checker/tests/expression_should_return_an_option.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/expression_should_return_an_option.vv:28:10: error: expression should return an option
+   26 |     }
+   27 |     // should be an checker error:
+   28 |     if x := return_string() {
+      |             ~~~~~~~~~~~~~~~
+   29 |         println('x: $x')
+   30 |     }

vlib/v/checker/tests/expression_should_return_an_option.vv

@@ -0,0 +1,31 @@
+fn return_optional(fail bool) ?string {
+	if fail {
+		return error('nope')
+	}
+	return 'foo'
+}
+
+fn return_string() string {
+	return 'foo'
+}
+
+fn main() {
+	// works
+	if r := return_optional(false) {
+		println(r)
+	}
+	// works
+	if r := return_optional(false) {
+		println(r)
+	} else {
+		println(err)
+	}
+	// works
+	return_optional(true) or {
+		println(err)
+	}
+	// should be an checker error:
+	if x := return_string() {
+		println('x: $x')
+	}
+}