Flexible Constructor Bodies (JEP 513) allows assignments to fields in an early construction context, with some restrictions. We didn't anticipate one extra necessary restriction: assignments should not happen before an alternate constructor invocation.

This is a problem mainly because of future language evolution:

- We'd like to support early reads of fields, implemented via proxy locals. But a proxy local won't be preserved across a constructor invocation, so the assigned value will be lost.

- We'd like to support early references to fields with initializers, where initializers run "first" in the construction process; but the earliest initializers can be run is the start of a super-calling constructor, where they will overwrite whatever was written in a this-calling constructor.

- We'd like to support user-defined default values for non-nullable types, but again these initial values can't be written until the start of a super-calling constructor, overwriting whatever was written in a this-calling constructor.

The right model for users is that the prologue of a this-calling constructor exists at a uniquely sensitive point of construction—you can set up parameters to pass to another constructor, but you can't interact at all yet with the current object.