Add tighter bounds on AnyNamedTuple

[soronpo]

AnyNamedTuple should have a Tuple lower bound and a Product upper-bound

[sjrd]

We cannot change the right hand side anymore. It breaks binary and TASTy compatibility.

[WojciechMazur]

If we assume 3.6.0 and 3.6.1 do no exist, then we're still in the RC period where modifications are allowed. We're currently at 3.6.2-RC2, and it's probably the last moment before signatures would be written in stone forever.

[sjrd]

Regardless, if we are making such a change at this point, it means named tuples are not ready for prime time.

So either we back out and make them experimental again, or we don't touch them except for bug fixes.

[odersky]

Which makes me realize that we need another strategy to roll out new features. Hardly anybody uses them when they are experimental. And when we stabilize small nits like this one inevitably pop up. Which means we have only three choices:

• we accept that it's imperfect and freeze anyway
• we roll back, which means that we cannot get anything substantial into the language anymore, ever
• we accept some breakage to fix nits such as this one, for a limited period.

I am not sure what to do here. Java has previews, would that help?

[odersky]

How important is the change? AnyNamedTuple was intended as a marker "only named tuples can go here". It was not supposed to have any functionality beyond that.