fix(parser): safely omit empty concat literals

src/Parser/BinaryOpTrait.php

@@ -264,17 +264,17 @@ trait BinaryOpTrait
         $this->flattenConcatExpr($expr, $items);
 
         $argList = $prefixExpressions;
-        foreach ($items as $index => $item) {
-            // 在字符串拼接中，除了第一个元素之外，剩下的字符串如果出现空字符串，忽略这个字符串
-            if ($index > 0 && $item->value == '') {
+        foreach ($items as $item) {
+            // Keep one operand so concat still performs PHP string coercion.
+            // Prefix expressions are operands too (for example, the left-hand
+            // value of `.=`), so an empty RHS literal can be omitted there.
+            if ($argList !== [] && $this->isScalarString($item) && $item->value === '') {
                 continue;
             }
 
-            $argList[] = $this->prepareConcatOperand($this->parseExprAsValue($item), $this->detectTypeOfExpr($item));
-        }
-
-        if (sizeof($argList) == 1) {
-            return $argList[0];
+            $type = $this->detectTypeOfExpr($item);
+            $parsed = $this->parseExprAsValue($item);
+            $argList[] = $this->prepareConcatOperand($parsed, $type);
         }
 
         return Symbol::concat() . '({' . implode(', ', $argList) . '})';

tests/compiler/concat_empty.phpt

@@ -5,8 +5,29 @@ Concat empty strings
 function main() {
     var_dump('' . 1 . '' . 'a' . '');
     var_dump('' . 1);
+    var_dump(1 . '');
+    var_dump(false . '');
+    var_dump('' . '' . '');
+
+    $value = 'value';
+    var_dump('' . $value . '');
+    var_dump('' . get_value() . '');
+
+    $number = 1;
+    $number .= '';
+    var_dump($number);
+}
+
+function get_value(): string {
+    return 'called';
 }
 ?>
 --EXPECT--
 string(2) "1a"
 string(1) "1"
+string(1) "1"
+string(0) ""
+string(0) ""
+string(5) "value"
+string(6) "called"
+string(1) "1"