Indicate Drupal 8.4.x is not compatible with PHP7.2

The one downside of doing this is if someone is patching core to be compatible they'll have to have a patch to reverse this change too. The advantage of course is that someone doesn't waste time learning that 8.4.x is not PHP7.2 compatible.

Hmm... this would only work for composer-only workflows where you try to get the entire drupal/core package. If you have the codebase already and just do a composer install it won't error. This is because of https://github.com/wikimedia/composer-merge-plugin/issues/96 - if you run composer update --lock it breaks as expected but it is unlikely people are doing that before running composer.install.

Ah of course we need to update the lock file. Then it works... after doing this running composer install on PHP7.2 does:

Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - This package requires php >=5.5.9 <7.2 but your PHP version (7.2.0) does not satisfy that requirement.

I'm still ambivalent about the patch... but at least it works. tbh honest if you look at a Drupal 8.4.x site on PHP 7.2 you can see it is pretty broken.

Yep it is tempting to leave this can of worms alone. Just tagging so all release managers can be aware.

It is actually possible to do a clean install of standard with 8.4.x on PHP 7.2 if you have warnings disabled. The first hard break I encountered was on node/add/article - therefore, if we choose to do this, I think we need to add a system requirement too to prevent install and to put a big red error on admin/reports/status.

+++ b/core/includes/bootstrap.inc
@@ -24,6 +24,11 @@
+/**
+ * Maximum supported version of PHP for Drupal 8.4.x and lower.
+ */
+const DRUPAL_MAXIMUM_PHP = '7.1.999';

Let's not introduce a constant otherwise we have to maintain it for all time. At the moment we do not know the maximum PHP for 8.5.x for example.

@mondrake to me all of this feels like a bad idea. If we do what you propose then even to make Drupal 8 work PHP 7.3 we have to patch it. Even PHPUnit with it's aggressive changing of support for PHP versions doesn't do this.