Default type parameters are not in the language reference

[diwic]

Default type parameters (e g struct Foo<A = u32>(A)) is a language feature, but they are not mentioned in the language reference.

It would be nice if such a reference also mentioned the tricky semantics around how it interacts with inference, as mentioned here.

[bluss]

Type parameter defaults are syntactical at the moment and don't really interact with inference.

The linked issue is basically that given Test::<>::default() I expected that to be equivalent to <Test<>>::default() but it is equivalent to <Test<_, _>>::default()

[diwic]

Maybe it's a bit poorly phrased, what I meant was something like, if you change Test::<u32> to Test::<u32,_>, then you disable default type parameters and enable inference (?). And to enable default type parameters in the first place you need either the <Test> syntax (which I hadn't seen before today), or Test::<_>, but the latter is impossible to use for the first default type parameter (given the linked issue).

All this might be a poor layman's explanation, which is the reason I filed the bug in the hope of getting a better one :-)

Side note: What would have been more intuitive to me, would be an algorithm like:

1. If the type parameter is explicitly mentioned (i e Test<u32, &str>), use that
2. else if inference can find out the correct type, use that
3. else if there is a default type parameter, use that
4. else give a compiler error.

...i e, that they would interact more than they do today. But that's a bigger question (and one the might be hard to do in a backwards compatible fashion, I don't know).

[steveklabnik]

The reference lives in its own repository now, I've moved it to rust-lang/reference#24

Thanks!