Add validate_theme_requirements filter

src/wp-includes/theme.php

@@ -1002,7 +1002,20 @@ function validate_theme_requirements( $stylesheet ) {
 );
 }
 
-return true;
+/**
+ * Filters the theme requirement validation response.
+ *
+ * If a theme fails due to a Core-provided validation (incompatible WP, PHP versions), this
+ * filter will not fire. A WP_Error response will already be returned.
+ *
+ * This filter is intended to add additional validation steps by site administrators.
+ *
+ * @since 6.9.0
+ *
+ * @param bool|WP_Error $met_requirements True if the theme meets requirements, WP_Error if not.
+ * @param string $stylesheet Directory name for the theme.
+ */
+return apply_filters( 'validate_theme_requirements', true, $stylesheet );
 }
 
 /**

tests/phpunit/tests/theme.php

@@ -1469,4 +1469,48 @@ public function test_switch_to_blog_uses_original_template_path() {
 
 $this->assertSame( $template_path, $new_template_path, 'Switching blogs switches the template path' );
 }
+
+/**
+ * Verify the validate_theme_requirements theme responds as expected for twentyten.
+ */
+public function test_validate_theme_requirements_filter_default() {
+// Default expectation since twentyten has the least strict requirements.
+$this->assertTrue( validate_theme_requirements( 'twentyten' ) );
+}
+
+/**
+ * Verify that a filtered failure of validate_theme_requirements returns WP_Error
+ */
+public function test_validate_theme_requirements_filter_error() {
+// Adds an extra requirement that always fails.
+add_filter(
+'validate_theme_requirements',
+function () {
+return new WP_Error( 'theme_test_failed_requirement' );
+}
+);
+
+$this->assertInstanceOf( 'WP_Error', validate_theme_requirements( 'twentyten' ) );
+}
+
+/**
+ * Verify that the theme is passed through to the validate_theme_requirements filter by selectively erroring.
+ */
+public function test_validate_theme_requirements_filter_selective_failure() {
+// Adds an extra requirement only for a particular theme.
+add_filter(
+'validate_theme_requirements',
+function( $met_requirements, $stylesheet ) {
+if ( 'twentytwenty' === $stylesheet ) {
+return new WP_Error( 'theme_test_failed_requirement' );
+}
+return $met_requirements;
+},
+10,
+2
+);
+
+$this->assertTrue( validate_theme_requirements( 'twentyten' ) );
+$this->assertInstanceOf( 'WP_Error', validate_theme_requirements( 'twentytwenty' ) );
+}
 }