[wp-trac] [WordPress Trac] #29618: Improve `is_child_theme` to check for a valid Template value in child stylesheet comment header

[WordPress Trac]

#29618: Improve `is_child_theme` to check for a valid Template value in child
stylesheet comment header
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 Reporter:  lancewillett  |      Owner:
     Type:  enhancement   |     Status:  new
 Priority:  normal        |  Milestone:  Awaiting Review
Component:  Themes        |    Version:
 Severity:  normal        |   Keywords:  has-patch
  Focuses:                |
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 Currently `is_child_theme()` only verifies that template and stylesheet
 values don't match.

 It'd be more accurate and safer to also verify that the child theme
 stylesheet also has a valid `Template: [parentslug]` value in it.

 It's possible in situations where something goes wrong with a WP site or
 its options for template and stylesheet to not match without a child
 active. This extra check would avoid possible errors in cases like that,
 as well.

 See a bit of history in #12998 and #20546.

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Ticket URL: <https://core.trac.wordpress.org/ticket/29618>
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