tests: add checker and passing test for generic receiver indexing (#2…

…0036)

vlib/v/checker/tests/generic_mut_struct_index_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/generic_mut_struct_index_err.vv:7:3: error: type `mut Heap` does not support slicing
+    5 | 
+    6 | fn (mut h Heap[T]) insert(mut element T, pos int) {
+    7 |     h[pos] = element
+      |      ~~~~~
+    8 | }
+    9 |

vlib/v/checker/tests/generic_mut_struct_index_err.vv

@@ -0,0 +1,17 @@
+pub struct Heap[T] {
+	mut:
+	arr []&T
+}
+
+fn (mut h Heap[T]) insert(mut element T, pos int) {
+	h[pos] = element
+}
+
+fn main() {
+	mut h := &Heap[int]{
+		arr: []&int{cap: 10}
+	}
+	mut a := 3
+	h.insert(mut &a, 0)
+	dump(h)
+}

vlib/v/tests/generic_with_ptr_as_param_test.v

@@ -0,0 +1,15 @@
+struct Number[T] {
+	value T
+}
+
+fn f[T](numbers &Number[T], idx int) T {
+	return unsafe { numbers[idx].value }
+}
+
+fn test_indexing_a_pointer_to_generic_instances() {
+	numbers := [10]Number[int]{init: Number[int]{
+		value: index * 10
+	}}
+	assert f(&numbers[0], 3) == 30
+	assert f(&numbers[0], 9) == 90
+}