Fixes assertion on calling String builtin with symbol (#2803)

JerryScript-DCO-1.0-Signed-off-by: legendecas legendecas@gmail.com

jerry-core/ecma/builtin-objects/ecma-builtin-string.c

@@ -22,6 +22,9 @@
 #include "ecma-helpers.h"
 #include "ecma-objects.h"
 #include "ecma-string-object.h"
+#ifndef CONFIG_DISABLE_ES2015_SYMBOL_BUILTIN
+#include "ecma-symbol-object.h"
+#endif /* !CONFIG_DISABLE_ES2015_SYMBOL_BUILTIN */
 #include "ecma-try-catch-macro.h"
 #include "jrt.h"
 
@@ -109,6 +112,9 @@ ecma_builtin_string_object_from_char_code (ecma_value_t this_arg, /**< 'this' ar
 /**
  * Handle calling [[Call]] of built-in String object
  *
+ * See also:
+ *          ECMA-262 v6, 21.1.1.1
+ *
  * @return ecma value
  */
 ecma_value_t
@@ -119,10 +125,19 @@ ecma_builtin_string_dispatch_call (const ecma_value_t *arguments_list_p, /**< ar
 
   ecma_value_t ret_value = ECMA_VALUE_EMPTY;
 
+  /* 1. */
   if (arguments_list_len == 0)
   {
     ret_value = ecma_make_magic_string_value (LIT_MAGIC_STRING__EMPTY);
   }
+#ifndef CONFIG_DISABLE_ES2015_SYMBOL_BUILTIN
+  /* 2.a */
+  else if (ecma_is_value_symbol (arguments_list_p[0]))
+  {
+    ret_value = ecma_get_symbol_descriptive_string (arguments_list_p[0]);
+  }
+#endif /* !CONFIG_DISABLE_ES2015_SYMBOL_BUILTIN */
+  /* 2.b */
   else
   {
     ret_value = ecma_op_to_string (arguments_list_p[0]);

tests/jerry/es2015/symbol-prototype-tostring.js

@@ -29,6 +29,8 @@ try {
 
 var foo = Symbol ('foo');
 assert (foo.toString () === "Symbol(foo)");
+assert (String (foo) === "Symbol(foo)");
 
 var fooObj = Object (foo);
 assert (fooObj.toString () === "Symbol(foo)");
+assert (String (fooObj) === "Symbol(foo)");