Make impredicativity work properly

This ticket is a placeholder for work on impredicativity. In fact, with the new typechecker we have most of the story for impredicativity in place, in the style of QML. Still missing are:

• Rigid type signatures
• Notation for type application

Trac metadata

Trac field	Value
Version	6.12.3
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Blocking
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Operating system
Architecture

changed weight to 5

added Ttask Trac import labels

changed milestone to %7.2.1

changed milestone to %7.6.1

changed weight to 3

Trac metadata

Trac field	Value
Priority	normal → low

changed milestone to %7.6.2

changed milestone to %7.10.1

Moving to 7.10.1.

removed milestone

Moving to 7.12.1 milestone; if you feel this is an error and should be addressed sooner, please move it back to the 7.10.1 milestone.

changed milestone to %8.0.1

Milestone renamed

added ImpredicativeTypes label

added Plow label

I believe this has been subsumed by the recent Quick Look work, the implementation of which is being tracked at #18126. Closing.

closed