Allow arbitrary expressions for empty()

This change is as per RFC https://wiki.php.net/rfc/empty_isset_exprs.

The change allows passing the result of function calls and other
expressions to the empty() language construct. This is accomplished by
simply rewriting empty(expr) to !expr.

The change does not affect the suppression of errors when using empty()
on variables. empty($undefinedVar) will continue not to throw errors.
When an expression is used inside empty() on the other hand, errors will
not be suppressed. Thus empty($undefinedVar + $somethingElse) *will*
throw a notice.

The change also does not make empty() into a real function, so using
'empty' as a callback is still not possible.

In addition to the empty() changes the commit adds nicer error messages
when isset() is used on function call results or other expressions.

NEWS

@@ -7,6 +7,9 @@ PHP                                                                        NEWS
   . World domination
   . Improve set_exception_handler while doing reset.(Laruence)
   . Support constant array/string dereferencing. (Laruence)
+  . Add support for using empty() on the result of function calls and
+    other expressions (https://wiki.php.net/rfc/empty_isset_exprs).
+    (Nikita Popov)
 
 - Core:
   . Fixed bug #61681 (Malformed grammar). (Nikita Popov, Etienne, Laruence).

UPGRADING

@@ -28,6 +28,9 @@ PHP X.Y UPGRADE NOTES
 
 - Support constant array/string dereferencing. (Laruence)
   (https://wiki.php.net/rfc/constdereference)
+- Add support for using empty() on the result of function calls and
+  other expressions. Thus it is now possible to write empty(getArray()),
+  for example. (https://wiki.php.net/rfc/empty_isset_exprs)
 
 ========================================
 2. Changes in SAPI modules
@@ -51,7 +54,7 @@ PHP X.Y UPGRADE NOTES
   - Implemented format character "Z": NUL-padded string
   - "a" now does not remove trailing NUL characters on unpack() anymore
   - "A" will now strip all trailing ASCII whitespace on unpack() (it used to
-    remove only trailing spaces.
+    remove only trailing spaces)
 
 ========================================
 5. New Functions

Zend/tests/empty_with_expr.phpt

@@ -0,0 +1,32 @@
+--TEST--
+empty() with arbitrary expressions
+--FILE--
+<?php
+
+function getEmptyArray() { return []; }
+function getNonEmptyArray() { return [1, 2, 3]; }
+
+var_dump(empty([]));
+var_dump(empty([1, 2, 3]));
+
+var_dump(empty(getEmptyArray()));
+var_dump(empty(getNonEmptyArray()));
+
+var_dump(empty([] + []));
+var_dump(empty([1, 2, 3] + []));
+
+var_dump(empty("string"));
+var_dump(empty(""));
+var_dump(empty(true));
+var_dump(empty(false));
+--EXPECT--
+bool(true)
+bool(false)
+bool(true)
+bool(false)
+bool(true)
+bool(false)
+bool(false)
+bool(true)
+bool(false)
+bool(true)

Zend/tests/isset_expr_error.phpt

@@ -0,0 +1,8 @@
+--TEST--
+Error message for isset(func())
+--FILE--
+<?php
+isset(1 + 1);
+?>
+--EXPECTF--
+Fatal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead) in %s on line %d

Zend/tests/isset_func_error.phpt

@@ -0,0 +1,8 @@
+--TEST--
+Error message for isset(func())
+--FILE--
+<?php
+isset(abc());
+?>
+--EXPECTF--
+Fatal error: Cannot use isset() on the result of a function call (you can use "null !== func()" instead) in %s on line %d

Zend/zend_compile.c

@@ -6089,7 +6089,16 @@ void zend_do_isset_or_isempty(int type, znode *result, znode *variable TSRMLS_DC
 
 	zend_do_end_variable_parse(variable, BP_VAR_IS, 0 TSRMLS_CC);
 
-	zend_check_writable_variable(variable);
+	if (zend_is_function_or_method_call(variable)) {
+		if (type == ZEND_ISEMPTY) {
+			/* empty(func()) can be transformed to !func() */
+			zend_do_unary_op(ZEND_BOOL_NOT, result, variable TSRMLS_CC);
+		} else {
+			zend_error(E_COMPILE_ERROR, "Cannot use isset() on the result of a function call (you can use \"null !== func()\" instead)");
+		}
+
+		return;
+	}
 
 	if (variable->op_type == IS_CV) {
 		last_op = get_next_op(CG(active_op_array) TSRMLS_CC);

Zend/zend_language_parser.y

@@ -1158,6 +1158,7 @@ encaps_var_offset:
 internal_functions_in_yacc:
 		T_ISSET '(' isset_variables ')' { $$ = $3; }
 	|	T_EMPTY '(' variable ')'	{ zend_do_isset_or_isempty(ZEND_ISEMPTY, &$$, &$3 TSRMLS_CC); }
+	|	T_EMPTY '(' expr_without_variable ')' { zend_do_unary_op(ZEND_BOOL_NOT, &$$, &$3 TSRMLS_CC); }
 	|	T_INCLUDE expr 			{ zend_do_include_or_eval(ZEND_INCLUDE, &$$, &$2 TSRMLS_CC); }
 	|	T_INCLUDE_ONCE expr 	{ zend_do_include_or_eval(ZEND_INCLUDE_ONCE, &$$, &$2 TSRMLS_CC); }
 	|	T_EVAL '(' expr ')' 	{ zend_do_include_or_eval(ZEND_EVAL, &$$, &$3 TSRMLS_CC); }
@@ -1166,8 +1167,13 @@ internal_functions_in_yacc:
 ;
 
 isset_variables:
-		variable 				{ zend_do_isset_or_isempty(ZEND_ISSET, &$$, &$1 TSRMLS_CC); }
-	|	isset_variables ',' { zend_do_boolean_and_begin(&$1, &$2 TSRMLS_CC); } variable { znode tmp; zend_do_isset_or_isempty(ZEND_ISSET, &tmp, &$4 TSRMLS_CC); zend_do_boolean_and_end(&$$, &$1, &tmp, &$2 TSRMLS_CC); }
+		isset_variable			{ $$ = $1; }
+	|	isset_variables ',' { zend_do_boolean_and_begin(&$1, &$2 TSRMLS_CC); } isset_variable { zend_do_boolean_and_end(&$$, &$1, &$4, &$2 TSRMLS_CC); }
+;
+
+isset_variable:
+		variable				{ zend_do_isset_or_isempty(ZEND_ISSET, &$$, &$1 TSRMLS_CC); }
+	|	expr_without_variable	{ zend_error(E_COMPILE_ERROR, "Cannot use isset() on the result of an expression (you can use \"null !== expression\" instead)"); }
 ;
 
 class_constant: