Scala 3 Given/Using Transition: Implicit Conversion Chaining Unresolved

The Resolution of Conversion Chaining

In Scala 3, implicit conversion chaining is prohibited. Unlike Scala 2.13, where the compiler could apply multiple implicit conversions in sequence to resolve a type mismatch, Scala 3 limits the resolution to a single conversion step. If your migration from Scala 2.13 relies on a chain (e.g., Type A → Type B → Type C), the code will fail to compile in Scala 3 regardless of whether you use implicit def or given syntax.

Behavioral Differences and Assumptions

It is important to distinguish between implicit parameters (type classes) and implicit conversions. While given and using are the modern replacements for implicit parameters, they do not change the fundamental restriction on conversion chaining. The following behaviors apply to Scala 3.x:

• Resolution Depth: Only one conversion is applied. If the target type is not reached in one step, the compiler will not search for a second conversion to bridge the remaining gap.
• Macro-Generated Implicits: Implicits generated via macros are expanded into standard definitions. They are subject to the same single-conversion limit as manually written code.
• Priority: Scala 3 uses a more deterministic priority system (favoring local given instances over imported ones), which may result in a different conversion being selected if multiple candidates exist.

Migration Steps for Chained Conversions

To preserve functionality during your transition, you must flatten your conversion logic:

1. Identify Chains: Search for locations where a type is converted through an intermediate type to reach a target.
2. Create Direct Conversions: Define a direct implicit conversion from the source type to the final target type.
3. Refactor to Extension Methods: Where possible, replace implicit conversions with extension methods to make the transformation explicit and avoid the pitfalls of the implicit resolution algorithm.

Verification and Diagnostics

To verify if your code is relying on prohibited chaining, you can attempt a minimal reproduction in a Scala 3 environment:

// This will fail in Scala 3
implicit def aToB(a: A): B = ???
implicit def bToC(b: B): C = ???

def process(c: C) = ???
process(new A()) // Error: No implicit conversion from A to C

Required Diagnostic: To provide a more specific refactoring path, please clarify if your macro-generated conversions are intended to provide type class instances (which can be nested via using) or type transformations (which are subject to the single-conversion limit).