Exclude undefined from the PrimitiveType in grammar

[EdgarChen]

Closes #1139.

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[domenic]

This also removes undefined from ConstType, which I think is incorrect.

[EdgarChen]

but

The type of a constant (matching ConstType) must not be any type other than a primitive type. If an identifier is used, it must reference a typedef whose type is a primitive type.

and undefined is not a primitive type in

The primitive types are bigint, boolean and the numeric types.

So could a constant type be undefined?

[domenic]

It seems the intention was to allow that, per

undefined constant values in IDL are represented with the undefined token.

Probably we should just make undefined a primitive type? It's certainly a primitive type in JavaScript, and I'm not sure why we'd depart.

[petervanderbeken]

It seems the intention was to allow that, per

undefined constant values in IDL are represented with the undefined token.

And yet https://webidl.spec.whatwg.org/#prod-ConstValue has:

ConstValue ::
     BooleanLiteral
     FloatLiteral
     integer

[petervanderbeken]

Also, are there actual use cases for constants with undefined as the value?

It's certainly a primitive type in JavaScript, and I'm not sure why we'd depart.

So are String and Symbol, but unless there's a clear use case I don't think we would add those either?

[domenic]

Ah yeah. Actually allowing const undefined = undefined seems pretty dumb.

What's not dumb is allowing undefined as a DefaultValue.

[petervanderbeken]

Ah, yes, good point. So let's add it to DefaultValue (which actually has string too ;-)).

[petervanderbeken]

Note that DefaultValue is also used for dictionary members, which can't be undefined, so we might need some prose excluding it. Although undefined is not explicitly in:

When a boolean literal token (true or false), the null token, an integer token, a decimal token, one of the three special floating point literal values (Infinity, -Infinity or NaN), a string token, the two token sequence [], or the two token sequence {} is used as the default value, it is interpreted in the same way as for an operation’s optional argument default value.

so maybe that's good enough?

[EdgarChen]

I add undefined into DefaultValue (thanks for the patch from @petervanderbeken).