Why doesn't Magento 2.1 support PHP 7.0.3 to 7.0.5?

No official information but this Github issue gives a hint:

https://github.com/magento/magento2/issues/4070

alankent commented on 8 Apr

Could you try 7.0.4? There are reports of something breaking with 7.0.5. Not sure who's fault, but 7.0.5 is behaving differently in a way that impacts Magento.

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Vinai commented on 9 Apr

@alankent Same result in PHP 7.0.4

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moleman commented 9 days ago

I had the same issue when running on php 7.0.5 but it works on php 7.0.6

Seems like they preferred to skip compatibility for the broken patch releases of PHP instead of working around them.

Also https://github.com/magento/magento2/issues/2596

maksek commented 9 days ago

[...] we are not recommending using 7.0.5 due the bugs, check PHP version section - http://devdocs.magento.com/guides/v2.0/install-gde/system-requirements.html. Can you try to downgrade or upgrade PHP from 7.0.5 and check again?

From that doc page:

PHP

• 5.6.x

• 5.5.x, where x is 22 or greater

• 7.0.2–7.0.6 except for 7.0.5 (supported by Magento version 2.0.1 and later only)

There is a known PHP issue that affects our code compiler when using PHP 7.0.5. We recommend you not use PHP 7.0.5; instead, use PHP 7.0.2–7.0.4 or 7.0.6.

This does not mention problems with 7.0.3 or even 7.0.4 but it's likely that there were similar issues.