Fcall optimization: Avoid FETCH_DIM_R with UNUSED op2

ext/opcache/Optimizer/optimize_func_calls.c

@@ -112,6 +112,13 @@ void optimize_func_calls(zend_op_array *op_array, zend_optimizer_ctx *ctx)
 						opline->extended_value &= ZEND_FETCH_TYPE_MASK;
 						opline->opcode -= 9;
 					} else {
+						if (opline->opcode == ZEND_FETCH_DIM_FUNC_ARG
+								&& opline->op2_type == IS_UNUSED) {
+							/* FETCH_DIM_FUNC_ARG supports UNUSED op2, while FETCH_DIM_R does not.
+							 * Performing the replacement would create an invalid opcode. */
+							break;
+						}
+
 						opline->extended_value &= ZEND_FETCH_TYPE_MASK;
 						opline->opcode -= 12;
 					}

ext/opcache/tests/optimize_func_calls_001.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Don't create FETCH_DIM_R with UNUSED op2
+--FILE--
+<?php
+
+// Order matters
+test($arr[]);
+function test($arg) {}
+
+?>
+--EXPECTF--
+Fatal error: Uncaught Error: Cannot use [] for reading in %s:%d
+Stack trace:
+#0 {main}
+  thrown in %s on line %d