v.checker: restrict `x T` in `fn test<T>(x T){ unsafe{ println(x[0]) }`, but allow `x &T`

vlib/v/checker/checker.v

@@ -6510,10 +6510,16 @@ pub fn (mut c Checker) index_expr(mut node ast.IndexExpr) ast.Type {
 				break
 			}
 			.any {
+				gname := typ_sym.name
 				typ = c.unwrap_generic(typ)
 				node.left_type = typ
 				typ_sym = c.table.get_final_type_symbol(typ)
+				if typ.is_ptr() {
 					continue
+				} else {
+					c.error('generic type $gname does not support indexing, pass an array, or a reference instead, e.g. []$gname or &$gname',
+						node.pos)
+				}
 			}
 			else {
 				break

vlib/v/checker/tests/generic_param_used_as_an_array_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/generic_param_used_as_an_array_err.vv:2:10: error: generic type T does not support indexing, pass an array, or a reference instead, e.g. []T or &T
+    1 | fn test<T>(arr T) {
+    2 |     a := arr[1]
+      |             ~~~
+    3 |     println(a)
+    4 | }

vlib/v/checker/tests/generic_param_used_as_an_array_err.vv

@@ -0,0 +1,12 @@
+fn test<T>(arr T) {
+	a := arr[1]
+	println(a)
+}
+
+fn main() {
+	a := [1, 2, 3]!
+	test(a)
+
+	b := ['a', 'b', 'c']!
+	test(b)
+}