ScratchVar: Disallow @Subroutine recursion

[michaeldiamant]

Problem

Unintentionally, #198 opens the door to unintended interactions between ScratchVar and recursive @Subroutines. #230 explores the topic.

We must either implement an improvement or revert #198 to avoid releasing software with known unintended behavior.

Solution

Fail TEAL source compilation when a recursive @Subroutine containing a ScratchVar parameter exists.

Rationale:

• Since DynamicScratchVar is a new abstraction and we don't know how pervasively it'll be used, the ticket requests constraining usage until we get feedback otherwise.
• We discussed these alternatives and deemed the benefit-vs-cost tradeoff to be weaker than the proposed solution:
  • Support for recursion.
  • Add guard rails around allowed recursion cases.

Additionally:

• Add an inline comment explaining that no technical barrier exists to supporting DynamicScratchVar: User-defined DynamicScratchVars in @Subroutines produce opaque errors #232, though we're opting to defer support. Goal is to help future maintainers understand why the constraint exists.

Dependencies

N/A

Urgency

High - master cannot be safely tagged.

[tzaffi]

#261 resolves this IMO

[tzaffi]

this has been released https://github.com/algorand/pyteal/releases/tag/v0.11.0