Can't override a function in posts only - wordpress

Question

I'm working on a blog using the theme Coral Dark. This theme has support for post thumbnails, and I want to remove them from the individual posts and leave them everywhere else (home, archives, categories, etc). The thumbnails are created with a pluggable function, so in theory I should just have to override it. If I add this to the functions.php of my child theme:
function coral_dark_post_thumbnail() {}
The thumbnails effectively disappear. However, if I use a conditional to run the function in posts only:
if ( is_single() ) {
function coral_dark_post_thumbnail() {}
}
It does nothing. The thumbnails still appear everywhere. I can even replace is_single with anything else because WordPress simply ignores it. And if I add a negation to the conditional like this:
if ( ! is_single() ) {
function coral_dark_post_thumbnail() {}
}
The thumbnails disappear but everywhere. Again, I can replace is_single with whatever because the result is the same. I don't understand what's happening.
This is the original function:
if ( ! function_exists( 'coral_dark_post_thumbnail' ) ) :
/**
* Displays an optional post thumbnail.
*
* Wraps the post thumbnail in an anchor element on index views, or a div
* element when on single views.
*
* Create your own coral_dark_post_thumbnail() function to override in a child theme.
*
*/
function coral_dark_post_thumbnail() {
if ( post_password_required() || is_attachment() || ! has_post_thumbnail() ) {
return;
}
if ( is_singular() ) :
?>
<div class="post-thumbnail">
<?php the_post_thumbnail('large'); ?>
</div><!-- .post-thumbnail -->
<?php else : ?>
<a class="post-thumbnail" href="<?php the_permalink(); ?>" aria-hidden="true">
<?php the_post_thumbnail( 'post-thumbnail', array( 'class' => 'alignleft smallpostthumb', 'alt' => the_title_attribute( 'echo=0' ), 'sizes' => '(max-width: 480px) 100vw, 210px' ) ); ?>
</a>
<?php endif; // End is_singular()
}
endif;
Many thanks in advance. :)

Answer 1

Seems your problem is with the condition. You cannot use condition to wrap your function li
if ( is_single() ) {
function coral_dark_post_thumbnail() {}
}
Instead use:
function coral_dark_post_thumbnail() {
if ( is_single() ) {
//TODO: Something
}
}
Doing the above should work.