When you want to get the language name of a language you can type:

{{ inputLanguage | language_name }}

But if inputLanguage is not a valid language, a MissingResourceException is thrown, blowing up the whole page.

Since in Twig, there is no way to catch this exception, it becomes extremely hard to work with these helpers if you have no control over the input. There is also no way to first check if the language exists before invoking the filter.

The same applies to all the other Intl:: calls that happen. They can all throw the same exception. I didn't change that yet, because I first want to know your opinion about this. If you agree, that it should fallback to the input, then I will change the rest too.

This is just to show some weird behaviour that I have no idea how to fix it.

What I've found so far is that the problem seems to occur when the currency symbol ¤ is involved in the text attributes:

https://github.com/twigphp/intl-extra/blob/ae0757394e8fc86ee2d0a8398f5947e9ac7c27d5/src/IntlExtension.php#L411-L413

When numberFormatterProto is created with en locale these are its properties before (in red) and after (in green) going through the IntlExtension:

2,3c2,3
<   locale: "en"
<   pattern: "¤#,##0.0"
---
>   locale: "en_US"
>   pattern: "'¤'#,##0.0;'-¤'#,##0.0"
18c18
<     FORMAT_WIDTH: false
---
>     FORMAT_WIDTH: 0

In case of fr locale are these:

2,3c2,3
<   locale: "fr"
<   pattern: "#,##0.0 ¤"
---
>   locale: "en_US"
>   pattern: "#,##0.0 '¤';'-'#,##0.0 '¤'"
18c18
<     FORMAT_WIDTH: false
---
>     FORMAT_WIDTH: 0

Looks like it escapes the symbol and does not change it to the local currency.

And this is the PHPUnit output:

Testing Twig\Extra\Intl\Tests\IntlExtensionTest
FF                                                                  2 / 2 (100%)

Time: 121 ms, Memory: 6.00 MB

There were 2 failures:

1) Twig\Extra\Intl\Tests\IntlExtensionTest::testFormatterProtoWithCurrency with data set #0 ('en', '€12.3')
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'€12.3'
+'¤12.3'

/intl-extra/tests/IntlExtensionTest.php:37

2) Twig\Extra\Intl\Tests\IntlExtensionTest::testFormatterProtoWithCurrency with data set #1 ('fr', '12,3 €')
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'12,3 €'
+'12,3 ¤'

/intl-extra/tests/IntlExtensionTest.php:37

FAILURES!
Tests: 2, Assertions: 2, Failures: 2.

I originally described this issue in https://github.com/twigphp/Twig/issues/3568#issuecomment-931382860 but have created this PR to keep the discussion focussed.

This is a necessary dependency.

Otherwise, when upgrading symfony 5.0.* to 5.2.

The inability to run composer require twig/intl-extra.

P.s. Excuse my English, I'm learning Japanese.