Type parameters inside super() calls?

Maurizio Cimadamore maurizio.cimadamore at oracle.com
Thu Feb 2 10:12:32 UTC 2023

On 02/02/2023 01:04, Alex Buckley wrote:
> But you are saying this is "static context overreach", and that 
> perhaps `new Local()` should be allowed there. Such an expression 
> needs to use `this` as the immediately enclosing instance of the Local 
> object, and that's fine as long as no-one can access the immediately 
> enclosing instance ... but now we invoke a method on the Local object 
> and ???? 

I realized my local class example was borked in some ways, and not in 

It is borked because when you do `new Local()` you need an enclosing 
instance (`this`), but you have not yet ran `super`, so that's a clear 
issue (same accessing an instance method).

But, should the _type_ Local be unavailable? What if I do `(Local)null` 
- is that an error? Today it will be - but here the expression is not 
using `this` in any way.

What irks me is that using the "static context" trick biases us on which 
_types_ we can and cannot see, while the whole point of the restrictions 
is to prevent initialization bugs (e.g. use `this` before `super` is 
called) which involve _expressions_ not _types_.

Because of that, I find the type-variable restriction too over-reaching 
- if the only way to sensibly use a type-variable was by using `this`, 
fine, it wouldn't matter in practice. But a constructor can accept an 
argument whose type is a type-variable - to manipulate that type you 
never need to use `this`. And, more concretely, let's say you need to 
call a _static_ generic method and pass that argument (some T), and 
inference fails, then the user, with the current restrictions, has no 
way to "fix" the inference error by providing a concrete <T> witness. 
Again, I don't expect this to come up much, but it's one of those things 
that seems somewhat arbitrary.


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