Source

File: wp-admin/includes/class-core-upgrader.php

public static function should_update_to_version( $offered_ver ) {
		require ABSPATH . WPINC . '/version.php'; // $wp_version; // x.y.z

		$current_branch = implode( '.', array_slice( preg_split( '/[.-]/', $wp_version ), 0, 2 ) ); // x.y
		$new_branch     = implode( '.', array_slice( preg_split( '/[.-]/', $offered_ver ), 0, 2 ) ); // x.y

		$current_is_development_version = (bool) strpos( $wp_version, '-' );

		// Defaults:
		$upgrade_dev   = true;
		$upgrade_minor = true;
		$upgrade_major = false;

		// WP_AUTO_UPDATE_CORE = true (all), 'minor', false.
		if ( defined( 'WP_AUTO_UPDATE_CORE' ) ) {
			if ( false === WP_AUTO_UPDATE_CORE ) {
				// Defaults to turned off, unless a filter allows it.
				$upgrade_dev   = false;
				$upgrade_minor = false;
				$upgrade_major = false;
			} elseif ( true === WP_AUTO_UPDATE_CORE ) {
				// ALL updates for core.
				$upgrade_dev   = true;
				$upgrade_minor = true;
				$upgrade_major = true;
			} elseif ( 'minor' === WP_AUTO_UPDATE_CORE ) {
				// Only minor updates for core.
				$upgrade_dev   = false;
				$upgrade_minor = true;
				$upgrade_major = false;
			}
		}

		// 1: If we're already on that version, not much point in updating?
		if ( $offered_ver == $wp_version ) {
			return false;
		}

		// 2: If we're running a newer version, that's a nope.
		if ( version_compare( $wp_version, $offered_ver, '>' ) ) {
			return false;
		}

		$failure_data = get_site_option( 'auto_core_update_failed' );
		if ( $failure_data ) {
			// If this was a critical update failure, cannot update.
			if ( ! empty( $failure_data['critical'] ) ) {
				return false;
			}

			// Don't claim we can update on update-core.php if we have a non-critical failure logged.
			if ( $wp_version == $failure_data['current'] && false !== strpos( $offered_ver, '.1.next.minor' ) ) {
				return false;
			}

			/*
			 * Cannot update if we're retrying the same A to B update that caused a non-critical failure.
			 * Some non-critical failures do allow retries, like download_failed.
			 * 3.7.1 => 3.7.2 resulted in files_not_writable, if we are still on 3.7.1 and still trying to update to 3.7.2.
			 */
			if ( empty( $failure_data['retry'] ) && $wp_version == $failure_data['current'] && $offered_ver == $failure_data['attempted'] ) {
				return false;
			}
		}

		// 3: 3.7-alpha-25000 -> 3.7-alpha-25678 -> 3.7-beta1 -> 3.7-beta2.
		if ( $current_is_development_version ) {

			/**
			 * Filters whether to enable automatic core updates for development versions.
			 *
			 * @since 3.7.0
			 *
			 * @param bool $upgrade_dev Whether to enable automatic updates for
			 *                          development versions.
			 */
			if ( ! apply_filters( 'allow_dev_auto_core_updates', $upgrade_dev ) ) {
				return false;
			}
			// Else fall through to minor + major branches below.
		}

		// 4: Minor in-branch updates (3.7.0 -> 3.7.1 -> 3.7.2 -> 3.7.4).
		if ( $current_branch == $new_branch ) {

			/**
			 * Filters whether to enable minor automatic core updates.
			 *
			 * @since 3.7.0
			 *
			 * @param bool $upgrade_minor Whether to enable minor automatic core updates.
			 */
			return apply_filters( 'allow_minor_auto_core_updates', $upgrade_minor );
		}

		// 5: Major version updates (3.7.0 -> 3.8.0 -> 3.9.1).
		if ( version_compare( $new_branch, $current_branch, '>' ) ) {

			/**
			 * Filters whether to enable major automatic core updates.
			 *
			 * @since 3.7.0
			 *
			 * @param bool $upgrade_major Whether to enable major automatic core updates.
			 */
			return apply_filters( 'allow_major_auto_core_updates', $upgrade_major );
		}

		// If we're not sure, we don't want it.
		return false;
	}