"enum value does not fit" for uint64 enums

[iceboy233]

Example:

enum Foo : uint64 { a }

Result:

error: a.fbs:1: 23: error: enum value does not fit [0; -1]

Looks like this overflowed:

flatbuffers/src/idl_parser.cpp

Line 1677 in 7d3930a

int64_t min_value = static_cast<int64_t>( \

Introduced in 5c0f914.

[iceboy233]

At the same time we should also allow empty enums.

[vglavnyy]

flatbuffers::numeric_limits<CTYPE>::max()) returns 0xFFFFFFFF interpreted as -1.
The 0 is greater than -1.

This issue has a simple solution, but there is a hidden problem.
Internally, the underlying type of any enums is int64_t (see struct EnumVal).
Therefore it is impossible to use enum values large then max(long) even if the fbs-enum has ulong underlying type.
static_cast<int64_t> from uint64_t is implementation-defined (platform dependent) since C++03:

If the destination type is signed, the value is unchanged if it can be represented in the destination type > (and bit-field width); otherwise, the value is implementation-defined.

[aardappel]

Hah, that's unfortunate.. looks like we need a special case there?

[iceboy233]

I think we can split the current EnumVal into two parts:

struct EnumValBase {
  std::string name;
  std::vector<std::string> doc_comment;
  Type union_type;
};

template <typename T>
struct EnumVal {
  EnumValBase base;
  T value;
};

Not sure on the reflection side. I feel we can use union-like type there.

[aardappel]

I'm not sure how that helps, since the type of the enum is only known at runtime (in the parser).

I think the changes can be entirely in code. We can obtain the desired data with reinterpret_cast<uint64_t> or a union.

[iceboy233]

In this way, every code that needs to access EnumVal<T>::value needs to know T at compile time (by template param). We still need a runtime dispatch at some level, but I think that might already partially exist, like the current CTYPE.

[aardappel]

@iceb0y I don't think this can work.. Now EnumDef needs to be templated as well (since it contains EnumVals), and now we can't have a type for the list of EnumDefs in Parser since each is different. You'll end up dispatching and casting all over the place. Better to just cast the actual value storage.

[vglavnyy]

@iceb0y do you already have a solution?

I think it is acceptable to use simple static_cast<int64_t>(u64_val) to store a new enum value to the vector without type punning.

Implicit signed to unsigned cast is modulo 2n cast by the C++ standard.
But unsigned to signed cast is implementation-defined by the C++ standard if a u64 value doesn't fit int64.
De facto the unsigned to signed cast is modulo 2n too.
I hope, the C++20 standard will make this cast well-defined:
https://en.cppreference.com/w/cpp/language/implicit_conversion
Proposal:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0907r3.html

[iceboy233]

Looking at code I agree that it's better to dispatch at value level than higher level. We can continue to use int64_t to store the value, while casting from T to int64_t, we can branch on value > max(int64_t) to use defined behavior. For longer term we can introduce a type utility that makes this easier.

[vglavnyy]

@aardappel
I have got a draft for this issue.
The solution will be ready in one or two weeks.
Probably #5194 may depend on this issue.

[aardappel]

Thanks @vglavnyy !

[vglavnyy]

@aardappel what do you think about unsigned bit_flags?
Now, an enum with bit_flag attribute can have any integer underlying type.
Problem:
If the underlying type is int32 then bit_31 is less than bit_30 by the value.

In C++ it is possible to set "sign" bit using static_cast from an unsigned value or using an explicit negative value. For other languages, explicit negation may be the only solution.

In C++ there are no bit_flags like 1 << bit_pos. One should specify each flag explicitly:

enum test_i8 : int8_t {
 b0 = 1 << 0,
 b7_err = 1<< 7,     // this left-shift is UB
 b7_ok = -128 // well-defined
}

C# has [Flags] attribute, but also doesn't allow (1<<7):

[Flags]
enum test_i8 : sbyte
{
  b0 = 1 << 0,
  b7_err = 1 << 7, // compile time error, 128 is out of range
  b7_ok = -128      // valid
}

Can we limit the signed bif_flags by [1 << (sizeof(T)*8-2)] value to avoid complexity and ambiguity?
Grammar restriction: bit_flags end value shall be non-negative.

[vglavnyy]

Any enum values can be combined by OR:

flatbuffers/tests/test.cpp

Lines 1391 to 1407 in f93d0f6

	void EnumStringsTest() {
	flatbuffers::Parser parser1;
	TEST_EQ(parser1.Parse("enum E:byte { A, B, C } table T { F:[E]; }"
	"root_type T;"
	"{ F:[ A, B, \"C\", \"A B C\" ] }"),
	true);
	flatbuffers::Parser parser2;
	TEST_EQ(parser2.Parse("enum E:byte { A, B, C } table T { F:[int]; }"
	"root_type T;"
	"{ F:[ \"E.C\", \"E.A E.B E.C\" ] }"),
	true);
	}
	void EnumNamesTest() {
	TEST_EQ_STR("Red", EnumNameColor(Color_Red));
	TEST_EQ_STR("Green", EnumNameColor(Color_Green));
	TEST_EQ_STR("Blue", EnumNameColor(Color_Blue));

Grammar:

For enums representing flags, you may place multiple inside a string
separated by spaces to OR them, e.g.
field: "EnumVal1 EnumVal2" or field: "Enum.EnumVal1 Enum.EnumVal2".

What does "represent the flags" mean in the grammar?
Probably, this is the wrong expected values in the test.
Is this test correct for enums without bit_flags attribute?