Deprecate left-associative ternary

Deprecate nesting ternary operators without explicit parentheses.

RFC: https://wiki.php.net/rfc/ternary_associativity

Author: nikic

UPGRADING

@@ -27,6 +27,16 @@ PHP 7.4 UPGRADE NOTES
   . "fn" is now a reserved keyword. In particular it can no longer be used as a
     function or class name. It can still be used as a method or class constant
     name.
+  . Nesting ternary operators without explicit parentheses is deprecated:
+
+        // Code like
+        $a ? $b : $c ? $d : $e
+        // should be replaced by (current interpretation)
+        ($a ? $b : $c) ? $d : $e
+        // or (likely intended interpretation)
+        $a ? $b : ($c ? $d : $e)
+
+    RFC: https://wiki.php.net/rfc/ternary_associativity
 
 - Curl:
   . Attempting to serialize a CURLFile class will now generate an exception.

Zend/tests/bug72944.phpt

@@ -2,7 +2,7 @@
 Bug #72944 (Null pointer deref in zval_delref_p).
 --FILE--
 <?php
-"a"== e & $A = $A? 0 : 0 ?:0;
+("a"== e & $A = $A? 0 : 0) ?:0;
 echo "OK\n";
 ?>
 --EXPECTF--

Zend/tests/ternary_associativity.phpt

@@ -0,0 +1,30 @@
+--TEST--
+Using ternary associativity is deprecated
+--FILE--
+<?php
+
+1 ? 2 : 3 ? 4 : 5;   // deprecated
+(1 ? 2 : 3) ? 4 : 5; // ok
+1 ? 2 : (3 ? 4 : 5); // ok
+
+// While the associativity of ?: is also incorrect, it will not cause a
+// functional difference, only some unnecessary checks.
+1 ?: 2 ?: 3;   // ok
+(1 ?: 2) ?: 3; // ok
+1 ?: (2 ?: 3); // ok
+
+1 ?: 2 ? 3 : 4;   // deprecated
+(1 ?: 2) ? 3 : 4; // ok
+1 ?: (2 ? 3 : 4); // ok
+
+1 ? 2 : 3 ?: 4;   // deprecated
+(1 ? 2 : 3) ?: 4; // ok
+1 ? 2 : (3 ?: 4); // ok
+
+?>
+--EXPECTF--
+Deprecated: Unparenthesized `a ? b : c ? d : e` is deprecated. Use either `(a ? b : c) ? d : e` or `a ? b : (c ? d : e)` in %s on line 3
+
+Deprecated: Unparenthesized `a ?: b ? c : d` is deprecated. Use either `(a ?: b) ? c : d` or `a ?: (b ? c : d)` in %s on line 13
+
+Deprecated: Unparenthesized `a ? b : c ?: d` is deprecated. Use either `(a ? b : c) ?: d` or `a ? b : (c ?: d)` in %s on line 17

Zend/zend_compile.c

@@ -7373,6 +7373,30 @@ void zend_compile_conditional(znode *result, zend_ast *ast) /* {{{ */
 	zend_op *opline_qm_assign2;
 	uint32_t opnum_jmpz, opnum_jmp;
 
+	if (cond_ast->kind == ZEND_AST_CONDITIONAL
+			&& cond_ast->attr != ZEND_PARENTHESIZED_CONDITIONAL) {
+		if (cond_ast->child[1]) {
+			if (true_ast) {
+				zend_error(E_DEPRECATED,
+					"Unparenthesized `a ? b : c ? d : e` is deprecated. "
+					"Use either `(a ? b : c) ? d : e` or `a ? b : (c ? d : e)`");
+			} else {
+				zend_error(E_DEPRECATED,
+					"Unparenthesized `a ? b : c ?: d` is deprecated. "
+					"Use either `(a ? b : c) ?: d` or `a ? b : (c ?: d)`");
+			}
+		} else {
+			if (true_ast) {
+				zend_error(E_DEPRECATED,
+					"Unparenthesized `a ?: b ? c : d` is deprecated. "
+					"Use either `(a ?: b) ? c : d` or `a ?: (b ? c : d)`");
+			} else {
+				/* This case is harmless:  (a ?: b) ?: c always produces the same result
+				 * as a ?: (b ?: c). */
+			}
+		}
+	}
+
 	if (!true_ast) {
 		zend_compile_shorthand_conditional(result, ast);
 		return;

Zend/zend_compile.h

@@ -994,6 +994,9 @@ static zend_always_inline int zend_check_arg_send_type(const zend_function *zf,
 #define ZEND_ARRAY_NOT_PACKED		(1<<1)
 #define ZEND_ARRAY_SIZE_SHIFT		2
 
+/* Attribute for ternary inside parentheses */
+#define ZEND_PARENTHESIZED_CONDITIONAL 1
+
 /* For "use" AST nodes and the seen symbol table */
 #define ZEND_SYMBOL_CLASS    (1<<0)
 #define ZEND_SYMBOL_FUNCTION (1<<1)

Zend/zend_language_parser.y

@@ -961,7 +961,10 @@ expr:
 			{ $$ = zend_ast_create_binary_op(ZEND_SPACESHIP, $1, $3); }
 	|	expr T_INSTANCEOF class_name_reference
 			{ $$ = zend_ast_create(ZEND_AST_INSTANCEOF, $1, $3); }
-	|	'(' expr ')' { $$ = $2; }
+	|	'(' expr ')' {
+			$$ = $2;
+			if ($$->kind == ZEND_AST_CONDITIONAL) $$->attr = ZEND_PARENTHESIZED_CONDITIONAL;
+		}
 	|	new_expr { $$ = $1; }
 	|	expr '?' expr ':' expr
 			{ $$ = zend_ast_create(ZEND_AST_CONDITIONAL, $1, $3, $5); }