Add 'bitflags' modifier for enum.

[ShuiRuTian]

Fixes #42521

• Add modifier bitflags.
• bit operator(|,|=,&,&=,^,^=) is only allowed for bitflags enum, if left or right is enum.
• Add new flag to enable bitflags check.

The second one is a big break! Maybe it should be loosen, but how?

And I think there are more features could be added!

• bitflags enum member could only be initialized by BitwiseOperator(&,|,^) or ShiftOperator(<<,>>,>>>) or bit number

bitflags enum BitEnum {
    TS = 1,     // 1 is 1 << 0, OK
    TSX = 2,    // 2 is 1 << 1, OK
    ERR2 = "1", // "1" is not number, error
    All = TS | TSX, // initialized through `|`, should be OK without "1"
    All2 = 3,    // 3 is not a bit, error
    All3 = 1+2 // '+' is not  BitwiseOperator or ShiftOperator, error.
}

• more accurate type.

bitflags enum BitEnum {
    TS = 1,
    TSX = 2,
    TSAlias = TS,
    All = TS | TSX,
}
const foo1 = BitEnum.TSAlias;
const foo2 = BitEnum.TSX;
const foo3 = foo1 | foo2;    // what is foo1 type? `BitEnum.TS` or `BitEnum.TSAlias`?

• give accurate type when both bitflags enum are known.

bitflags enum BitEnum {
    TS = 1,
    TSX = 2,
    All = TS | TSX,
}

var foo1 = BitEnum.TS;
var foo2 = BitEnum.TSX;
var foo3 = foo1 | foo2; // foo3 is possiable to be BitEnum.All?
declare var foo4: BitEnum;
var foo5 = foo4 & foo1; // foo5 is possiable to be BitEnum.TS?

---

Some Questions:

1. What should be the flag name? Should it be "strict"? Personally, I want to enable this feature in any new project, but it might be a big break for current project. Now it is bitEnum
2. Is there a better name rather than bitflags?
3. How to mark enum as bit? Should bitflags be a modifier or tag or something else?
4. Are error messages easy to understand? I am clearly not a English native speaker, they might looks wired now.

[weswigham]

@ShuiRuTian we were discussing allowing the modifier/tag/whatever in any mode, but only having it do anything different from normal enums today under a new flag - so you can opt in to strictly checking bitflagginess. This way it wouldn't be a straight break for anyone currently using normal enums as bitflags.

[ShuiRuTian]

What is EnumKind on earth?

[ShuiRuTian]

@weswigham Thanks a lot! the beta version is ready for review.

But, I find something strange about the enums, which blocks me on more accurate type. item in the check list.

It is basically about EnumKind. I feel EnumKind.Numeric is something like number, and EnumKind.Literal is like string.

But at the same time, we could see in function getDeclaredTypeOfEnum, in the block we have also getLiteralType and getRegularTypeOfLiteralType. Literal for these two functions is more like Syntax.stringLiteral and Syntax.numericLiteral.

Also, it seems if type is EnumKind.Numeric, then enum members does not have any chance to have a correct declaredType -- if it is not initialized, in the second time it could only get its parent cached type(Not sure about this, but I am sure enum member in EnumKind.Numeric does not have as rich flags as those members in EnumKind.Literal).

---

PS: I change a big else-if to

switch(true){
case (condition):
...
}

Reason: this helps debug, the code would jump to the first true condition, pass a lot false condition.
But is this good or at least acceptable? Is this too tricky to make code hard to read?
Would revert them if you think this trade-off is not good.

[typescript-bot]

The TypeScript team hasn't accepted the linked issue #42521. If you can get it accepted, this PR will have a better chance of being reviewed.

[weswigham]

I feel EnumKind.Numeric is something like number, and EnumKind.Literal is like string

Numeric is the old bitflag allowing enum, while Literal is the newer union-of-uniqe-literal-subtypes enum. You get the later if the enum contains a string initializer today. You probably wanna make it so when the strict bitflag flag is set, all enums are Literal unless they have the bitflag modifier, at which point they're Numeric-ish.

[ShuiRuTian]

Seems a new keyword is not acceptable. Would restart working on this if there is any new thought.

[sandersn]

To help with PR housekeeping, I'm going to close this draft PR. It is pretty old now.