Revert "checker: check generic type indexing error (#10345)"

This reverts commit 103c777ad6.

Passing references to values should be allowed, and is already used in vinix:
`pub fn cas<T>(_here &T, _ifthis T, writethis T) bool {`

vlib/v/checker/checker.v

@@ -6493,23 +6493,32 @@ fn (mut c Checker) check_index(typ_sym &ast.TypeSymbol, index ast.Expr, index_ty
 
 pub fn (mut c Checker) index_expr(mut node ast.IndexExpr) ast.Type {
 	mut typ := c.expr(node.left)
-	typ_sym := c.table.get_final_type_symbol(typ)
+	mut typ_sym := c.table.get_final_type_symbol(typ)
 	node.left_type = typ
-	match typ_sym.kind {
+	for {
-		.map {
+		match typ_sym.kind {
-			node.is_map = true
+			.map {
+				node.is_map = true
+				break
+			}
+			.array {
+				node.is_array = true
+				break
+			}
+			.array_fixed {
+				node.is_farray = true
+				break
+			}
+			.any {
+				typ = c.unwrap_generic(typ)
+				node.left_type = typ
+				typ_sym = c.table.get_final_type_symbol(typ)
+				continue
+			}
+			else {
+				break
+			}
 		}
-		.array {
-			node.is_array = true
-		}
-		.array_fixed {
-			node.is_farray = true
-		}
-		.any {
-			c.error('generic type `$typ_sym.name` does not support indexing, please use compound type, e.g. `[]$typ_sym.name`',
-				node.pos)
-		}
-		else {}
 	}
 	if typ_sym.kind !in [.array, .array_fixed, .string, .map] && !typ.is_ptr()
 		&& typ !in [ast.byteptr_type, ast.charptr_type] && !typ.has_flag(.variadic) {

vlib/v/checker/tests/generics_type_para_index_err.out

@@ -1,6 +0,0 @@
-vlib/v/checker/tests/generics_type_para_index_err.vv:2:24: error: generic type `T` does not support indexing, please use compound type, e.g. `[]T`
-    1 | fn show_element<T>(arr &T) string {
-    2 |     return unsafe { '${arr[1]}' }
-      |                           ~~~
-    3 | }
-    4 |

vlib/v/tests/generics_with_fixed_array_type_test.v

@@ -2,8 +2,9 @@ fn show_element<T>(arr &T) string {
 	return unsafe { '${arr[1]}' }
 }
 
-fn main() {
+fn test_generic_with_fixed_array_type() {
 	a := [1, 2, 3]!
 	ret := show_element(a)
 	println(ret)
+	assert ret == '2'
 }