The Scala type-checking process distorts certain ASTs (such as representations of extractors or case classes) in a way that they cannot be type-checked again. The issue is described in SI-5464. Ideally, it should be possible to 1) type-check an AST, 2) transform the AST and 3) re-type-check the transformed AST. This does not work for all ASTs using the typecheck and untypecheck methods provided by the Scala reflection library.

On a Context c, retypecheck provides the methods c.retyper.typecheck, c.retyper.untypecheck (based on standard c.untypecheck), c.retyper.untypecheckAll (based on c.resetAllAttrs of the resetallattrs library), c.retyper.retypecheck (using retyper.untypecheck followed by retyper.typecheck), c.retyper.retypecheckAll (using retyper.untypecheckAll followed by retyper.typecheck). These wrapper methods around the type-checking/un-type-checking process try to fix up the AST in a way that it can be type-checked again on a best effort basis. They do not aim at undoing all of the compiler's desugerings during type-checking.

Add the dependency to your build.sbt:

libraryDependencies += "io.github.scala-loci" %% "retypecheck" % "0.10.0"

import retypecheck._

def impl(c: Context)(...) = {
  ...
  c.retyper.typecheck(...)
  c.retyper.retypecheck(...)
  c.retyper.untypecheck(...)
  ...
}