gcc suppress warning "too small to hold all values of"

As I recall, an enum with a declared underlying type can hold any value of that type, regardless of what enumeration constants are defined. Since you can say

val= enum2{148}

and expect it to work properly, the warning seems correct for that case. You are not declaring a base type, and historically this means that the enum is only guaranteed to be big enough to hold the range of values given by the lowest through highest enumeration constant. So I would expect no warning here. Maybe the new enum class also expects a full range even though the underlying type was automatically determined (or the compiler thinks it does)? You might try using a pure old-syntax enum and see if that works any differently.

The problem is that an scoped enum always has an integral underlying type. By default, it is int but you can change it to any other integral type, such as unsigned char.

Unfortunately you cannot change the underlying type to a bit-field, as they are not real C++ types.

You could try disabling the warning, but a quick skim through the G++ code reveals these lines (gcc/cp/class.c:3468):

  else if (TREE_CODE (type) == ENUMERAL_TYPE
           && (0 > (compare_tree_int
                    (w, TYPE_PRECISION (ENUM_UNDERLYING_TYPE (type))))))
    warning_at (DECL_SOURCE_LOCATION (field), 0,
                "%qD is too small to hold all values of %q#T",
                field, type);

The key here is the call to warning_at(...) instead of warning(OPT_to_disable_the_warning, ...). So currently there is no option to disable it. Except recompiling the compiler yourself!

For what it is worth CLang++-3.7.1 does not warn about it.