Make sure arguments are evaluated in the correct typer state.

There's a tricky interaction with caching of typed arguments in FunProto types
and backtracking using different typer states. We might end up with a typed
argument that is evaluated in one typer state and that is used in another. The
problem is that the argument typing might have inserted type variables (maybe
by adding polymorphic implicit views) that are not registered in the typer
state in which the application is finally typed. In that case we will see
an "orphan poly parameter" in pickling.

The fix is to discard argument types is their typerstate is not committed
to the one in which the application is finally typed. To apply the fix we
need to track

 - for typer states: whether or not it was committed, and what its parent is.
 - for function prototypes: the typer state in which an argument with cached type
   was evaluated.

Test case is t1756.scala, which produced an "orphan poly parameter CI" before.

1 files changed, 3 insertions, 6 deletions

diff --git a/src/dotty/tools/dotc/typer/Typer.scala b/src/dotty/tools/dotc/typer/Typer.scala
index 34cd5448b..678e408e4 100644
--- a/src/dotty/tools/dotc/typer/Typer.scala
+++ b/src/dotty/tools/dotc/typer/Typer.scala
@@ -1445,11 +1445,8 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       val sel = typedSelect(untpd.Select(untpd.TypedSplice(tree), nme.apply), pt)
       if (sel.tpe.isError) sel else adapt(sel, pt)
     } { (failedTree, failedState) =>
-      tryInsertImplicitOnQualifier(tree, pt) match {
-        case Some(tree1) => adapt(tree1, pt)
-        case none => fallBack(failedTree, failedState)
-      }
-     }
+      tryInsertImplicitOnQualifier(tree, pt).getOrElse(fallBack(failedTree, failedState))
+    }
 
   /** If this tree is a select node `qual.name`, try to insert an implicit conversion
    *  `c` around `qual` so that `c(qual).name` conforms to `pt`. If that fails
@@ -1462,7 +1459,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         tryEither { implicit ctx =>
           val qual1 = adaptInterpolated(qual, qualProto, EmptyTree)
           if ((qual eq qual1) || ctx.reporter.hasErrors) None
-          else Some(typedSelect(cpy.Select(tree)(untpd.TypedSplice(qual1), name), pt))
+          else Some(typed(cpy.Select(tree)(untpd.TypedSplice(qual1), name), pt))
         } { (_, _) => None
         }
       case _ => None