In the following, the implicit is selected despite requiring a prefix because of the local shadowing.

class A
class B

trait S {
    implicit def aToB(a: A): B = new B
}

class T extends S {
    def x: B = {
        val aToB = 3
        // doesn't compile, because aToB method requires 'T.this.' prefix
        //aToB(new A)
        
        // compiles, using T.this.aToB,
        //   despite it not being accessible without a prefix
        new A
    }
}

As I understand it, Section 7.2 of the spec says that the first part of implicit resolution finds implicits accessible without a prefix. Ch. 2 says that a local identifier shadows an inherited identifier because it has the same precedence but is in an inner scope, which is why aToB(new A) doesn't compile. Therefore, S.aToB should not be considered for implicit resolution.

The same behavior occurs when defining aToB directly in T or importing an implicit named aToB (say, from an object P). Putting implicit in front of the local val aToB shows the expected behavior of not selecting the S.aToB implicit, giving a compile error.