move ExtractAPI after RefChecks

We need to make sure RefChecks run before ExtractAPI, as there
might be cyclic reference exceptions. See neg/i1750.scala.

Author: liufengyun

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -44,7 +44,6 @@ class Compiler {
       List(new FrontEnd),           // Compiler frontend: scanner, parser, namer, typer
       List(new sbt.ExtractDependencies), // Sends information on classes' dependencies to sbt via callbacks
       List(new PostTyper),          // Additional checks and cleanups after type checking
-      List(new sbt.ExtractAPI),     // Sends a representation of the API of classes to sbt via callbacks
       List(new Pickler),            // Generate TASTY info
       List(new FirstTransform,      // Some transformations to put trees into a canonical form
            new CheckReentrant),     // Internal use only: Check that compiled program has no data races involving global vars
@@ -57,6 +56,7 @@ class Compiler {
            new TailRec,             // Rewrite tail recursion to loops
            new LiftTry,             // Put try expressions that might execute on non-empty stacks into their own methods
            new ClassOf),            // Expand `Predef.classOf` calls.
+      List(new sbt.ExtractAPI),     // Sends a representation of the API of classes to sbt via callbacks
       List(new TryCatchPatterns,    // Compile cases in try/catch
            new PatternMatcher,      // Compile pattern matches
            new ExplicitOuter,       // Add accessors to outer classes from nested ones.

compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala

@@ -35,7 +35,7 @@ class ExtractAPI extends Phase {
   // after `PostTyper` (unlike `ExtractDependencies`, the simplication to trees
   // done by `PostTyper` do not affect this phase because it only cares about
   // definitions, and `PostTyper` does not change definitions).
-  override def runsAfter = Set(classOf[transform.PostTyper])
+  override def runsAfter = Set(classOf[transform.PostTyper], classOf[typer.RefChecks])
 
   override def run(implicit ctx: Context): Unit = {
     val unit = ctx.compilationUnit