cgen: properly cast to sumtypes in array prepend and insert (#11289)

vlib/v/gen/c/array.v

@@ -411,7 +411,7 @@ fn (mut g Gen) gen_array_insert(node ast.CallExpr) {
 		if left_info.elem_type == ast.string_type {
 			g.write('string_clone(')
 		}
-		g.expr(node.args[1].expr)
+		g.expr_with_cast(node.args[1].expr, node.args[1].typ, left_info.elem_type)
 		if left_info.elem_type == ast.string_type {
 			g.write(')')
 		}
@@ -441,7 +441,7 @@ fn (mut g Gen) gen_array_prepend(node ast.CallExpr) {
 		g.write('.len)')
 	} else {
 		g.write(', &($elem_type_str[]){')
-		g.expr(node.args[0].expr)
+		g.expr_with_cast(node.args[0].expr, node.args[0].typ, left_info.elem_type)
 		g.write('})')
 	}
 }

vlib/v/tests/array_of_sumtypes_test.v

@@ -0,0 +1,20 @@
+struct Node {
+mut:
+	tag     string
+	content []Content
+}
+
+struct Text {
+}
+
+type Content = Node | Text
+
+fn test_push_prepend_insert() {
+	mut body := Node{}
+	body.content << Node{
+		tag: 'a'
+	}
+	body.content.prepend(Node{ tag: 'b' })
+	body.content.insert(1, Node{ tag: 'c' })
+	assert body.content.len == 3
+}