checker: fix indexing a type alias instance (#7889)

vlib/v/checker/checker.v

@@ -4643,7 +4643,7 @@ fn (mut c Checker) check_index_type(typ_sym &table.TypeSymbol, index_type table.
 pub fn (mut c Checker) index_expr(mut node ast.IndexExpr) table.Type {
 	typ := c.expr(node.left)
 	node.left_type = typ
-	typ_sym := c.table.get_type_symbol(typ)
+	typ_sym := c.table.get_final_type_symbol(typ)
 	if typ_sym.kind !in [.array, .array_fixed, .string, .map] && !typ.is_ptr() && !(!typ_sym.name[0].is_capital() &&
 		typ_sym.name.ends_with('ptr')) && !typ.has_flag(.variadic) { // byteptr, charptr etc
 		c.error('type `$typ_sym.name` does not support indexing', node.pos)

vlib/v/gen/cgen.v

@@ -3846,7 +3846,7 @@ fn (mut g Gen) index_expr(node ast.IndexExpr) {
 			g.write(')')
 		}
 		else {
-			sym := g.table.get_type_symbol(node.left_type)
+			sym := g.table.get_final_type_symbol(node.left_type)
 			left_is_ptr := node.left_type.is_ptr()
 			if sym.kind == .array {
 				info := sym.info as table.Array

vlib/v/table/table.v

@@ -638,7 +638,7 @@ pub fn (mut t Table) add_placeholder_type(name string, language Language) int {
 
 [inline]
 pub fn (t &Table) value_type(typ Type) Type {
-	typ_sym := t.get_type_symbol(typ)
+	typ_sym := t.get_final_type_symbol(typ)
 	if typ.has_flag(.variadic) {
 		// ...string => string
 		// return typ.clear_flag(.variadic)

vlib/v/tests/array_type_alias_test.v

@@ -0,0 +1,6 @@
+type Test = []int
+
+fn test_index() {
+    t := Test([2,4])
+    assert t[1] == 4
+}

vlib/v/tests/map_type_alias_test.v

@@ -0,0 +1,6 @@
+type Test = map[string]string
+
+fn test_index() {
+    t := Test({'c': 'abc'})
+    assert t['c'] == 'abc'
+}