Arrow function are not allowed outside filters

[alex-dev]

Arrow functions currently can only be used in a filter as per ExpressionParser.
This prevents multiple functionalities such as

• passing arrows to an include context so it can forward it to an actual filter;
• passing arrows to a macro or a function;
• defining and setting a callable so it can be used in subsequent expressions;

[fabpot]

The limitations you list are on purpose to limit usage of arrow functions to simple use cases.

[khalwat]

To make this work this one line: https://github.com/twigphp/Twig/blob/3.x/src/ExpressionParser.php#L483 from:

                    foreach ($this->parseArguments() as $n) {

to:

                    foreach ($this->parseArguments(false, false, true) as $n) {

then closures all work in the context we want, e.g.:

    {% set collection = collect(['a', 'b', 'c']) %}
    {% set contains = collection.contains((value, key) => value == 'a') %}

with a nice, clean, normal syntax. I'd love to see this happen, too.

[fabpot]

See #4378