checker: allow casting strings to string aliases

vlib/v/ast/ast.v

@@ -703,7 +703,7 @@ pub struct CastExpr {
 pub:
 	expr      Expr // `buf`
 	arg       Expr // `n` in `string(buf, n)`
-	typ       table.Type // `string`
+	typ       table.Type // `string` TODO rename to `type_to_cast_to`
 	pos       token.Position
 pub mut:
 	typname   string

vlib/v/checker/checker.v

@@ -1962,6 +1962,8 @@ pub fn (mut c Checker) expr(node ast.Expr) table.Type {
 					node.pos)
 			}
 			if node.expr_type == table.string_type {
+				cast_to_type_sym := c.table.get_type_symbol(node.typ)
+				if cast_to_type_sym.kind != .alias {
 					mut error_msg := 'cannot cast a string'
 					if node.expr is ast.StringLiteral {
 						str_lit := node.expr as ast.StringLiteral
@@ -1971,6 +1973,7 @@ pub fn (mut c Checker) expr(node ast.Expr) table.Type {
 					}
 					c.error(error_msg, node.pos)
 				}
+			}
 			if node.has_arg {
 				c.expr(node.arg)
 			}