Fix checks related to value classes

[liufengyun]

Fix checks related to value classes: #1670 #1642

Note: despite the discussion today, the simplest fix is to move some RefChecks to typer. The TODO comment in the typer already points the direction for the migration:

+
+
+    // check value class constraints
+    RefChecks.checkDerivedValueClass(cls, body1)
+
     cdef1

     // todo later: check that
     //  1. If class is non-abstract, it is instantiatable:
     //  - self type is s supertype of own type
     //  - all type members have consistent bounds
     // 2. all private type members have consistent bounds
     // 3. Types do not override classes.
     // 4. Polymorphic type defs override nothing.

And I'd argue the complexity is manageable -- typer is not becoming more complex. And having checks earlier simplifies error handling and later processing.

Review @odersky .

[smarter]

No. That's completely fine and supported in Dotty. The only issue is when there is a cycle, this happens a value class underlying type (or the underlying type of its underlying type or ...) is the class itself. There is a checkNonCyclic method in ExtensionMethods that is supposed to do this but is currently disabled, try enabling it:
https://github.com/lampepfl/dotty/blob/master/src/dotty/tools/dotc/transform/ExtensionMethods.scala#L148-L150

[liufengyun]

Thanks for the catch @smarter , I'm going to enable that.

[odersky]

Otherwise LGTM

[odersky]

That's a drastic change. Why disable all checking in TreeChecker?

If it's just checkDerivedValue class that should be disabled during tree checking then it's better to
override that method directly.

[liufengyun]

@odersky this is now addressed in the latest commit.

[liufengyun]

/rebuild

[odersky]

LGTM