AlwaysReturnInFilter: differentiate between abstract method and parse…

… error

This implements the suggestion made in Automattic#580 (comment)

If the method a filter callback points to is an abstract method, a warning will be thrown asking for manual inspection of the child class implementations of the abstract method.

In case of parse or tokenizer error, the sniff will bow out and stay silent.

WordPressVIPMinimum/Sniffs/Hooks/AlwaysReturnInFilterSniff.php

@@ -175,12 +175,18 @@ private function processFunction( $stackPtr, $start = 0, $end = null ) {
 	 */
 	private function processFunctionBody( $stackPtr ) {
 
-		/**
-		 * Stop if the function doesn't have a body, like when it is abstract.
-		 *
-		 * @see https://github.com/squizlabs/PHP_CodeSniffer/blob/master/src/Standards/Generic/Sniffs/NamingConventions/ConstructorNameSniff.php#L87-L90
-		 */
-		if ( isset( $this->tokens[ $stackPtr ]['scope_closer'] ) === false ) {
+		$filterName = $this->tokens[ $this->filterNamePtr ]['content'];
+
+		$methodProps = $this->phpcsFile->getMethodProperties( $stackPtr );
+		if ( $methodProps['is_abstract'] === true ) {
+			$message = 'The callback for the `%s` filter hook-in points to an abstract method. Please, make sure that all child class implementations of this abstract method always return a value.';
+			$data    = [ $filterName ];
+			$this->phpcsFile->addWarning( $message, $stackPtr, 'AbstractMethod', $data );
+			return;
+		}
+
+		if ( isset( $this->tokens[ $stackPtr ]['scope_opener'], $this->tokens[ $stackPtr ]['scope_closer'] ) === false ) {
+			// Live coding, parse or tokenizer error.
 			return;
 		}
 
@@ -198,8 +204,6 @@ private function processFunctionBody( $stackPtr ) {
 			return;
 		}
 
-		$filterName = $this->tokens[ $this->filterNamePtr ]['content'];
-
 		$functionBodyScopeStart = $this->tokens[ $stackPtr ]['scope_opener'];
 		$functionBodyScopeEnd   = $this->tokens[ $stackPtr ]['scope_closer'];