Distinguishability of nullable types and dictionaries seems broken

[bzbarsky]

Consider this IDL:

void foo(DOMString? arg);
void foo(optional SomeDictionaryType arg);

Is this valid? I believe it shouldn't be. DOMString? and SomeDictionaryType should not be distinguishable. But as the spec stands right now, we have:

Two types are distinguishable if at most one of the two includes a nullable type and at least one of the following three conditions is true:

In this case, the first of the three conditions is true, since DOMString and SomeDictionaryType are distinguishable. Looking at the definition of "includes a nullable type" we have:

A type includes a nullable type if:
the type is a nullable type, or
the type is a union type and its number of nullable member types is 1.

In this case neither type is a union type. The definition of "nullable type" is:

A nullable type is an IDL type constructed from an existing type (called the inner type), which just allows the additional value null to be a member of its set of values. Nullable types are represented in IDL by placing a U+003F QUESTION MARK ("?") character after an existing type.

so only DOMString? is a nullable type and the two types are specced distinguishable.

This used to not be broken. For example, https://web.archive.org/web/20160801174701/https://heycam.github.io/webidl/ says:

Two types are distinguishable if at most one of the two includes a nullable type or is a dictionary type, and at least one of the following three conditions is true:

which makes these two types not distinguishable, since both are either nullable or dictionary....

[bzbarsky]

@tobie, @jyasskin, @domenic Oh, this got broken by #180 which removed the relevant important bit from the definition of "distinguishable"....

God, I wish we had unit tests for this spec. :(

[jyasskin]

Oh, wow, you parsed "at most one of the two includes a nullable type or is a dictionary type" differently than I did. You did «"includes a nullable type or is a dictionary type" is true of at most one of the two», while I did «at most one of the two includes a nullable type and at most one of the two is a dictionary type». I'll send you a patch.

[bzbarsky]

Yeah, I agree the original wording could have been clearer.....