PR #1551: Fixing Nested Modifier Propagation

Continuing my work on Quantinuum’s Guppy: I fixed a bug where the compiler silently accepted invalid quantum programs.

Guppy lets developers apply modifiers to blocks of quantum code — like making an operation conditional, reversing a circuit, or repeating it. Functions declare which modifiers they support, and the compiler should reject anything that violates those rules.

The bug: when modifier blocks were nested inside each other, the compiler only checked the innermost block’s rules and forgot about everything outside it. As a result, invalid code compiled without any error or warning.

In PR #1551, I fixed this by ensuring the compiler carries the full set of active rules into every nested block — so constraints are never lost, no matter how deep the nesting goes.

Fixes like this are what I find most rewarding — making the tools developers rely on more trustworthy. It’s my hope that this contribution helps Guppy programmers accelerate their work.

Thanks to the Guppy contributors at Quantinuum for their collaboration and thoughtful reviews.

For more technical details, refer to PR #1551.