template_preprocess() and other preprocess functions are called even for theme hooks implemented as functions

I don't understand how this would fix the bug?

+++ b/core/modules/system/tests/themes/test_basetheme/test_basetheme.theme
@@ -27,3 +27,9 @@ function test_basetheme_views_post_render(ViewExecutable $view) {
+function test_basetheme_preprocess_theme_test_function_suggestions($variables) {

Super duper minor and doesn't really apply here but &$variables :)

Looks good!

Only one other thing:

+++ b/core/modules/system/src/Tests/Theme/RegistryTest.php
@@ -99,6 +99,11 @@ public function testMultipleSubThemes() {
+    $preprocess_functions = $registry_base_theme->get()['theme_test_function_suggestions']['preprocess functions'];
+    $this->assertIdentical([
+      'test_basetheme_preprocess_theme_test_function_suggestions',
+    ], $preprocess_functions);

This should probably have an inline comment about what it's testing…

Fixing!

Fixing more stuff!

Adding some feedback from IRC so it's here too.

1. +++ b/core/lib/Drupal/Core/Theme/Registry.php
   @@ -456,12 +456,14 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
   -        if (isset($info['function']) && !function_exists($info['function'])) {
   -          throw new \BadFunctionCallException(sprintf(
   -            'Theme hook "%s" refers to a theme function callback that does not exist: "%s"',
   -            $hook,
   -            $info['function']
   -          ));
   +        if (isset($info['function'])) {
   +          if (!function_exists($info['function'])) {
   +            throw new \BadFunctionCallException(sprintf(
   +                'Theme hook "%s" refers to a theme function callback that does not exist: "%s"',
   +                $hook,
   +                $info['function']
   +              ));
   +          }
   

   Might be clearer for the elseif after this to instead do:

   if (!isset($info['function']) && !isset($info['template'])) {

2. +++ b/core/modules/system/src/Tests/Theme/RegistryTest.php
   @@ -99,6 +99,11 @@ public function testMultipleSubThemes() {
   +
   +    $preprocess_functions = $registry_base_theme->get()['theme_test_function_suggestions']['preprocess functions'];
   +    $this->assertIdentical([
   +      'test_basetheme_preprocess_theme_test_function_suggestions',
   +    ], $preprocess_functions, "Theme functions doesn't have template preprocess function");
   
   +++ b/core/modules/system/tests/themes/test_basetheme/test_basetheme.theme
   @@ -27,3 +27,9 @@ function test_basetheme_views_post_render(ViewExecutable $view) {
   +
   +/**
   + * Implements hook_preprocess_HOOK() for theme_test_function_suggestions theme functions.
   + */
   +function test_basetheme_preprocess_theme_test_function_suggestions(&$variables) {
   +}
   

   I think we should add in the theme_test module (or wherever) template_preprocess_theme_test_function_suggestions() to make sure that still works.

   Because that's what didn't work with lauriii's approach and is still pretty important. We just don't want the template_preprocess() function itself running for theme functions.

Found this when playing with bootstrap which does some altering on theme registry, which eventually broke all theme hook that use functions.

Patch looks good to me. I agree if/elseif should be pretty clear and self-explanatory.

Thank you for fixing this. Great job!

Meh... I noticed something else.

+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -456,12 +456,14 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
+                'Theme hook "%s" refers to a theme function callback that does not exist: "%s"',
+                $hook,
+                $info['function']
+              ));
+          }

Indentation seems to be a bit off here.

I was discussing this issue with @slashrsm around bootstrap and patch looks good.

+1, thanks!

+1 for RTBC

Committed/pushed to 8.0.x, thanks!