Added error message for union values out of range.

Change-Id: I481afcde6a554d1cad519ff95acac7f38a7f4ee5
Tested: on Linux.

src/idl_parser.cpp

@@ -1340,6 +1340,10 @@ CheckedError Parser::ParseEnum(bool is_union, EnumDef **dest) {
             enum_def.vals.vec[prevsize - 1]->value >= ev.value)
           return Error("enum values must be specified in ascending order");
       }
+      if (is_union) {
+        if (ev.value < 0 || ev.value >= 256)
+          return Error("union enum value must fit in a ubyte");
+      }
       if (opts.proto_mode && Is('[')) {
         NEXT();
         // ignore attributes on enums.

tests/test.cpp

@@ -1010,6 +1010,7 @@ void ErrorTest() {
   TestError("enum X:float {}", "underlying");
   TestError("enum X:byte { Y, Y }", "value already");
   TestError("enum X:byte { Y=2, Z=1 }", "ascending");
+  TestError("union X { Y = 256 }", "must fit");
   TestError("enum X:byte (bit_flags) { Y=8 }", "bit flag out");
   TestError("table X { Y:int; } table X {", "datatype already");
   TestError("struct X (force_align: 7) { Y:int; }", "force_align");