zend_compile: Allow (void) in for’s initializer and loop expression (php#18303)

* zend_compile: Allow `(void)` in for’s initializer and loop expression

The initializer and loop expression of a `for()` loop only accept expressions,
but they act like statements in spirit - their results are completely ignored.
Allow `(void)` there to allow suppressing `#[\NoDiscard]`, since there is no
semantic ambiguity of what `(void)` returns anyways.

Fixes php#18301

* zend_language_parser: Simplify `for()` grammar

Author: TimWolla

Zend/tests/type_casts/gh18301_cast_to_void_for.phpt

@@ -0,0 +1,46 @@
+--TEST--
+GH-18301: casting to void is allowed in for’s expression lists
+--FILE--
+<?php
+
+$count = 0;
+
+#[NoDiscard]
+function incCount() {
+	global $count;
+	$count++;
+	return $count;
+}
+
+for ( $count = 0, (void)incCount(), incCount(); (void)incCount(), incCount() < 30; incCount(), $count++, incCount(), (void)incCount()) {
+	echo $count . "\n";
+}
+
+?>
+--EXPECTF--
+Warning: The return value of function incCount() should either be used or intentionally ignored by casting it as (void) in %s on line %d
+4
+
+Warning: The return value of function incCount() should either be used or intentionally ignored by casting it as (void) in %s on line %d
+
+Warning: The return value of function incCount() should either be used or intentionally ignored by casting it as (void) in %s on line %d
+10
+
+Warning: The return value of function incCount() should either be used or intentionally ignored by casting it as (void) in %s on line %d
+
+Warning: The return value of function incCount() should either be used or intentionally ignored by casting it as (void) in %s on line %d
+16
+
+Warning: The return value of function incCount() should either be used or intentionally ignored by casting it as (void) in %s on line %d
+
+Warning: The return value of function incCount() should either be used or intentionally ignored by casting it as (void) in %s on line %d
+22
+
+Warning: The return value of function incCount() should either be used or intentionally ignored by casting it as (void) in %s on line %d
+
+Warning: The return value of function incCount() should either be used or intentionally ignored by casting it as (void) in %s on line %d
+28
+
+Warning: The return value of function incCount() should either be used or intentionally ignored by casting it as (void) in %s on line %d
+
+Warning: The return value of function incCount() should either be used or intentionally ignored by casting it as (void) in %s on line %d

Zend/tests/type_casts/gh18301_cast_to_void_statement_for_condition.phpt

@@ -0,0 +1,9 @@
+--TEST--
+GH-18301: casting to void is not allowed at the end of a for condition
+--FILE--
+<?php
+
+for (;(void)true;);
+?>
+--EXPECTF--
+Parse error: syntax error, unexpected token ";", expecting "," in %s on line %d

Zend/zend_compile.c

@@ -5717,6 +5717,26 @@ static void zend_compile_return(zend_ast *ast) /* {{{ */
 }
 /* }}} */
 
+static void zend_compile_void_cast(znode *result, zend_ast *ast)
+{
+	zend_ast *expr_ast = ast->child[0];
+	znode expr_node;
+	zend_op *opline;
+
+	zend_compile_expr(&expr_node, expr_ast);
+
+	switch (expr_node.op_type) {
+		case IS_TMP_VAR:
+		case IS_VAR:
+			opline = zend_emit_op(NULL, ZEND_FREE, &expr_node, NULL);
+			opline->extended_value = ZEND_FREE_VOID_CAST;
+			break;
+		case IS_CONST:
+			zend_do_free(&expr_node);
+			break;
+	}
+}
+
 static void zend_compile_echo(zend_ast *ast) /* {{{ */
 {
 	zend_op *opline;
@@ -5967,7 +5987,7 @@ static void zend_compile_do_while(zend_ast *ast) /* {{{ */
 }
 /* }}} */
 
-static void zend_compile_expr_list(znode *result, zend_ast *ast) /* {{{ */
+static void zend_compile_for_expr_list(znode *result, zend_ast *ast) /* {{{ */
 {
 	zend_ast_list *list;
 	uint32_t i;
@@ -5984,7 +6004,13 @@ static void zend_compile_expr_list(znode *result, zend_ast *ast) /* {{{ */
 		zend_ast *expr_ast = list->child[i];
 
 		zend_do_free(result);
-		zend_compile_expr(result, expr_ast);
+		if (expr_ast->kind == ZEND_AST_CAST_VOID) {
+			zend_compile_void_cast(NULL, expr_ast);
+			result->op_type = IS_CONST;
+			ZVAL_NULL(&result->u.constant);
+		} else {
+			zend_compile_expr(result, expr_ast);
+		}
 	}
 }
 /* }}} */
@@ -5999,7 +6025,7 @@ static void zend_compile_for(zend_ast *ast) /* {{{ */
 	znode result;
 	uint32_t opnum_start, opnum_jmp, opnum_loop;
 
-	zend_compile_expr_list(&result, init_ast);
+	zend_compile_for_expr_list(&result, init_ast);
 	zend_do_free(&result);
 
 	opnum_jmp = zend_emit_jump(0);
@@ -6010,11 +6036,11 @@ static void zend_compile_for(zend_ast *ast) /* {{{ */
 	zend_compile_stmt(stmt_ast);
 
 	opnum_loop = get_next_op_number();
-	zend_compile_expr_list(&result, loop_ast);
+	zend_compile_for_expr_list(&result, loop_ast);
 	zend_do_free(&result);
 
 	zend_update_jump_target_to_next(opnum_jmp);
-	zend_compile_expr_list(&result, cond_ast);
+	zend_compile_for_expr_list(&result, cond_ast);
 	zend_do_extended_stmt();
 
 	zend_emit_cond_jump(ZEND_JMPNZ, &result, opnum_start);
@@ -10594,26 +10620,6 @@ static void zend_compile_include_or_eval(znode *result, zend_ast *ast) /* {{{ */
 }
 /* }}} */
 
-static void zend_compile_void_cast(znode *result, zend_ast *ast)
-{
-	zend_ast *expr_ast = ast->child[0];
-	znode expr_node;
-	zend_op *opline;
-
-	zend_compile_expr(&expr_node, expr_ast);
-
-	switch (expr_node.op_type) {
-		case IS_TMP_VAR:
-		case IS_VAR:
-			opline = zend_emit_op(NULL, ZEND_FREE, &expr_node, NULL);
-			opline->extended_value = ZEND_FREE_VOID_CAST;
-			break;
-		case IS_CONST:
-			zend_do_free(&expr_node);
-			break;
-	}
-}
-
 static void zend_compile_isset_or_empty(znode *result, zend_ast *ast) /* {{{ */
 {
 	zend_ast *var_ast = ast->child[0];

Zend/zend_language_parser.y

@@ -268,7 +268,7 @@ static YYSIZE_T zend_yytnamerr(char*, const char*);
 %type <ast> callable_expr callable_variable static_member new_variable
 %type <ast> encaps_var encaps_var_offset isset_variables
 %type <ast> top_statement_list use_declarations const_list inner_statement_list if_stmt
-%type <ast> alt_if_stmt for_exprs switch_case_list global_var_list static_var_list
+%type <ast> alt_if_stmt for_cond_exprs for_exprs switch_case_list global_var_list static_var_list
 %type <ast> echo_expr_list unset_variables catch_name_list catch_list optional_variable parameter_list class_statement_list
 %type <ast> implements_list case_list if_stmt_without_else
 %type <ast> non_empty_parameter_list argument_list non_empty_argument_list property_list
@@ -508,7 +508,7 @@ statement:
 			{ $$ = zend_ast_create(ZEND_AST_WHILE, $3, $5); }
 	|	T_DO statement T_WHILE '(' expr ')' ';'
 			{ $$ = zend_ast_create(ZEND_AST_DO_WHILE, $2, $5); }
-	|	T_FOR '(' for_exprs ';' for_exprs ';' for_exprs ')' for_statement
+	|	T_FOR '(' for_exprs ';' for_cond_exprs ';' for_exprs ')' for_statement
 			{ $$ = zend_ast_create(ZEND_AST_FOR, $3, $5, $7, $9); }
 	|	T_SWITCH '(' expr ')' switch_case_list
 			{ $$ = zend_ast_create(ZEND_AST_SWITCH, $3, $5); }
@@ -1169,13 +1169,21 @@ echo_expr:
 	expr { $$ = zend_ast_create(ZEND_AST_ECHO, $1); }
 ;
 
+for_cond_exprs:
+		%empty			{ $$ = NULL; }
+	|	non_empty_for_exprs ',' expr { $$ = zend_ast_list_add($1, $3); }
+	|	expr { $$ = zend_ast_create_list(1, ZEND_AST_EXPR_LIST, $1); }
+;
+
 for_exprs:
 		%empty			{ $$ = NULL; }
 	|	non_empty_for_exprs	{ $$ = $1; }
 ;
 
 non_empty_for_exprs:
 		non_empty_for_exprs ',' expr { $$ = zend_ast_list_add($1, $3); }
+	|	non_empty_for_exprs ',' T_VOID_CAST expr { $$ = zend_ast_list_add($1, zend_ast_create(ZEND_AST_CAST_VOID, $4)); }
+	|	T_VOID_CAST expr { $$ = zend_ast_create_list(1, ZEND_AST_EXPR_LIST, zend_ast_create(ZEND_AST_CAST_VOID, $2)); }
 	|	expr { $$ = zend_ast_create_list(1, ZEND_AST_EXPR_LIST, $1); }
 ;