fix

vlib/v/checker/infix.v

@@ -756,7 +756,7 @@ fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 			right_expr := node.right
 			mut typ := match right_expr {
 				ast.TypeNode {
-					right_expr.typ
+					c.unwrap_generic(right_expr.typ)
 				}
 				ast.None {
 					ast.none_type_idx

vlib/v/tests/sumtypes/sumtype_generic_checking_test.v

@@ -0,0 +1,20 @@
+type Sumtype = string | int
+
+fn generic_fn[T]() ?T {
+	a := Sumtype('123')
+	if a is T {
+		return a
+	}
+
+	b := Sumtype(123)
+	if b is T {
+		return b
+	}
+
+	return none
+}
+
+fn test_main() {
+	assert generic_fn[string]().str() == "Option('123')"
+	assert generic_fn[int]().str() == 'Option(123)'
+}