Use strict comparison in various files

[SergeyBiryukov]

Trac ticket:
https://core.trac.wordpress.org/ticket/62279
https://core.trac.wordpress.org/ticket/61607
https://core.trac.wordpress.org/ticket/60700
https://core.trac.wordpress.org/ticket/58831
https://core.trac.wordpress.org/ticket/57839
https://core.trac.wordpress.org/ticket/56791

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Props sergeybiryukov, aristath, peterwilsoncc.

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[aristath]

LGTM 👍

[peterwilsoncc]

This will need tests before it can be merged:

To avoid a repeat of Core-61749:

• Anything that calls WP_Query will need checks for both arrays and query string formatted arguments passed to wp_parse_args()
• Same for any of the other altered functions that use wp_parse_args()

I can see an issue with the privacy policy code breaking is_privacy_policy for one but expect there will be other issue that need testing. To reproduce the issue with the privacy policy:

1. In the dashboard create a page named Privacy Policy
2. In Settings > Privacy define the page as such
3. Run the following command in the CLI

   $ git checkout trunk
   $ wp eval "var_dump( ( new WP_Query( [ 'page_id' => 128, 'post_type' => 'page' ] ) )->is_privacy_policy );"
   :bool(true)
   $ gh pr checkout 3873 ## This branch (Sergey can use git checkout)
   $ wp eval "var_dump( ( new WP_Query( [ 'page_id' => 128, 'post_type' => 'page' ] ) )->is_privacy_policy );"
   :bool(false)
   

[SergeyBiryukov]

This will need tests before it can be merged:

Indeed, this is basically still a draft where I cherry-pick certain changes after a closer review, making sure they are covered by new or existing tests. I will investigate anything related to wp_parse_args() and WP_Query, thanks!