Which Scala 3 using binding wins across compilation units

Answer

Compilation unit boundaries do not decide the winner. Scala 3 given/using resolution is scope-based, not file-based. A given that is defined in the local or lexically enclosing scope outranks a given that is brought in by an import, regardless of which source file each lives in. An enclosing class given therefore wins over an imported using group from another compilation unit.

Import proximity across files does not influence selection. If two imports from different compilation units both supply a matching given and neither shadows the other, the compiler reports an ambiguous given error instead of picking a winner.

Confirmed facts

• Resolution is tiered by scope, not by file. The first tier with an eligible candidate wins.
• Local / enclosing-scope givens are visible without a prefix and outrank imported givens.
• Imported givens are resolved within the import tier, with later and more specific imports shadowing earlier wildcard imports.
• Companion object implicit scope is a later tier than local and import tiers.
• The shadowed-using warning is a compile-time resolution fact. The chosen instance is fixed at the call site during compilation, not at runtime.
• Scala 2 implicit vals/defs participate in the same search when used together with Scala 3 givens. They are treated as regular implicit candidates, which can produce shadowing warnings when mixed with givens in the same scope.

Likely explanation

Nearest-scope first means lexical nesting, not file proximity. A given defined in a class, object, method or block that lexically encloses the call site is in a higher priority tier than any given that must be imported into that site, even if the imported given originates from the same file or a different compilation unit.

Given resolution happens per call site. The winner can differ between two call sites in the same file if their imports differ.

Steps needed for this case

1. Identify the lexical scope of the call site. If the call is inside the body of a class that defines a given for the required type, that given wins over any imported given.
2. Check the import form. A named import shadows a wildcard import within the import tier, but neither can outrank a local/enclosing definition.
3. Verify selection. Write a minimal two-file test with a distinguishing marker on each given and inspect which instance the compiler inserts at the call site. Compile with -Xprint:typer to see the implicit argument the compiler chose.
4. Test ambiguity. Import two different givens of the same type via wildcard imports without a local definition and confirm the compiler raises an ambiguous given error.

One missing diagnostic detail that changes the recommendation: the exact Scala 3 compiler version you are using and whether the enclosing given is defined in the enclosing class body itself versus in its companion object or a parent trait. Shadowing rules between named and wildcard imports have been refined across Scala 3 minor releases, and companion vs enclosing-class placement changes the tier.