Why is there an "object creation impossible error here"?

Question
1

Scala version - 2.13
Code

 def foo[A <: String]: Function1[String, Int] = new Function1[String, Int] {
    override def apply(a: A): Int = 1
  }

This give a compilation error

object creation impossible. Missing implementation for:
  def apply(v1: String): Int // inherited from trait Function1scalac

A is a subtype of String. Why is this not allowed?

2

The first type argument that you provided to Function1 is String but you defined the parameter a in apply to be of type A.

The error that I get is:

   error: object creation impossible. Missing implementation for:
     def apply(v1: String): Int // inherited from trait Function1
       override def apply(a: A): Int = 1
                    ^
On line 2: error: method apply overrides nothing.
       Note: the super classes of <$anon: String => Int> contain the following, non final members named apply:
       def apply(v1: String): Int

You need to specify A as the type argument to Function1 like so:

def foo[A <: String]: Function1[A, Int] = new Function1[A, Int] {
      override def apply(a: A): Int = 1
    }
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