checker: fix generic sumtype method (#11565)

vlib/v/checker/checker.v

@@ -2278,9 +2278,17 @@ pub fn (mut c Checker) method_call(mut node ast.CallExpr) ast.Type {
 		method = m
 		has_method = true
 	} else {
+		if left_type_sym.kind in [.struct_, .sum_type, .interface_] {
+			mut parent_type := ast.void_type
 			if left_type_sym.info is ast.Struct {
-			if left_type_sym.info.parent_type != 0 {
-				type_sym := c.table.get_type_symbol(left_type_sym.info.parent_type)
+				parent_type = left_type_sym.info.parent_type
+			} else if left_type_sym.info is ast.SumType {
+				parent_type = left_type_sym.info.parent_type
+			} else if left_type_sym.info is ast.Interface {
+				parent_type = left_type_sym.info.parent_type
+			}
+			if parent_type != 0 {
+				type_sym := c.table.get_type_symbol(parent_type)
 				if m := c.table.type_find_method(type_sym, method_name) {
 					method = m
 					has_method = true

vlib/v/tests/generic_sumtype_method_test.v

@@ -0,0 +1,32 @@
+struct Empty {}
+
+struct Node<T> {
+	value T
+	left  Leaf<T>
+	right Leaf<T>
+}
+
+type Leaf<T> = Empty | Node<T>
+
+// return size(number of nodes) of BST
+fn (leaf Leaf<T>) size<T>() int {
+	return match leaf {
+		Empty { 0 }
+		Node<T> { 1 + leaf.left.size() + leaf.right.size() }
+	}
+}
+
+fn test_generic_sumtype_method() {
+	r := Node<int>{
+		value: 20
+		left: Empty{}
+		right: Empty{}
+	}
+	tree := Leaf<int>(Node<int>{
+		value: 10
+		left: Empty{}
+		right: r
+	})
+	println(tree.size())
+	assert tree.size() == 2
+}