Here are some additional sources:
https://english.stackexchange.com/questions/2139/should-we-use-plural-or-singular-for-a-fraction-of-a-mile
https://en.wikipedia.org/wiki/Plural
http://mathforum.org/library/drmath/view/57224.html
http://www.antimoon.com/forum/t1070.htm

All I find tend to define 1.5 as plural in English but Symfony no longer do (it was the cas before the typing was added).

Your use case only used to work accidentally based on the fact that we were not able to use scalar type hints on older Symfony versions. The count value always was expected to be an integer. Decimals are currently not supported.

Before the typing, rules != 1 (English) and <2 (French) were accurate and able to work with floats. Nothing specified decimals were not supported and as far as I know, there were no bugs with floats. So despite it worked accidentally, forcing integer and limit the support to integers was actually a regression.

So unless float causes real problems. It should not be forbidden. If some pluralization rules does not support them, so those specific rules could have int typing, even if it would definitly better to have explicit floor() calls inside rules that need it and float typing instead.

Do you want me to propose a PR?

The interface defined that the number had to be an integer:

Since I have no idea if this works for all supported I would not be in favour of adding this feature.

You can put the int typing inside each plural rule except English which will have float, then change int to float in getPluralizationRule first argument. This way, you fix the English plural with no change in any other language.

Then it can be fixed language per language afterward and it will allow custom plural rule that would now be able to handle floating numbers.

Having correct plural in English seems an important concern for such a great project. Sounds a bit weird to say to symfony users: "Don't use decimal numbers if you wish a correct pluralization".

I have no idea if this works for all supported

Not a reason. This can be tested. I can add tests for 0.5, 1.5 and 2.5 in each language then fix the typing and see what languages changed, so we can check for each language in grammar reference if we have to fix it (as the English plural) or keep the previous rule by forcing int for this particular language.

Is this a regression or did it always work that way?
Since 4.2, we provide the intl+icu format for messages, which is able to deal with decimals properly.
Could you please give it a try?
See symfony/symfony-docs#10264

@nico-incubiq, it's the result for symfony 4.2, 1.5 liter, it must output 1.5 liters to be linguistically right. As long as you have the int typing, you won't be able to apply both correct French and English rules for decimals (and obviously many others). I will try with the version before the typing was added for comparison if I've some times.

For the record, plural in French starts at 2. So, it's "2 litres" but "1.5 litre". So, nothing is obvious when it comes to languages :)

Yes @fabpot it's exactly the point. Singular in French is -2 > x < 2, in English, it's x === -1 or x === 1.

Those rules include decimal numbers, so you can't be accurate using int typing.

NB: 1.0 miles is also plural while 1 mile is singular. But I guess this very particular case could be omitted.

The question is, does ICU define rules for floating numbers too ? If no, the ICU-based formatting would not help anyway and it is the new recommended default.

Yes it does (as long as you use groups "few, one, other" of course and not explicit values), this format is used in Angular, and Angular handle properly the plural of 1.5 in French and in English.

1.5 is "one" in French.
1.5 is "other" in English.

As simple.

Hey, thanks for your report!
There has not been a lot of activity here for a while. Is this bug still relevant? Have you managed to find a workaround?

No workaround possible, we sill have no way to properly pluralize sentences with floating numbers such as translating "1.5 liters" in English into "1.5 litre" in French.

I just tested this issue, with translations in ICU format, and I do not find the bug.

{{ 'product.carats'|trans({ 'carat': 1.2 }) }}

With this translation:

<unit id="zcajhWf" name="product.carats">
  <segment>
    <source>product.carats</source>
    <target>{carat, plural,
      one   {Weight: # carat}
      other {Weight: # carats}
    }</target>
  </segment>
</unit>

The output is correct:

Weight: 1.2 carats

The buggy code is still there:

You need to pass through this method to get the bug and you need 2 languages with different floating pluralization rules.

As long as int $number is in the pluralization rule, 1.2 is rounded down to 1 and so you can't have the proper pluralization for both a language where 1.2 has the same plural as 1 (like French) and an other language where it has the same plural as 2 (like English).

@kylekatarnls would you mind working on a fix please? That's a bugfix for branch 4.4.

Yes, you can assign it to me :)

Fix #39887 submitted.