Avoid double context creation when modes change on a fresh context.

Move addMode and friends to two decorators, one for Context, the other
for FreshContext. Implement behavior accordingly.

This avoids creating two contexts in situations like:

     c.fresh.setxploreTyperState.addMode(...)

Mow we can write

     c.fresh.addMode(...).setExploreTyperState

Because addMode returns a fresh context when applied to a fresh context.
Note that we specifically do not want virtual dispatch of addMode, that's
why it was moved to a decorator.

Also: removed mention of ".fresh: when just forllowed by an addMode, because
that one is redundant.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/dotty/tools/dotc/typer/Implicits.scala b/src/dotty/tools/dotc/typer/Implicits.scala
index 43d93b2b8..be76553be 100644
--- a/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/src/dotty/tools/dotc/typer/Implicits.scala
@@ -90,7 +90,7 @@ object Implicits {
       }
 
       if (refs.isEmpty) refs
-      else refs filter (refMatches(_)(ctx.fresh.setExploreTyperState.addMode(Mode.TypevarsMissContext))) // create a defensive copy of ctx to avoid constraint pollution
+      else refs filter (refMatches(_)(ctx.fresh.addMode(Mode.TypevarsMissContext).setExploreTyperState)) // create a defensive copy of ctx to avoid constraint pollution
     }
   }
 
@@ -480,7 +480,8 @@ trait Implicits { self: Typer =>
             pt)
         val generated1 = adapt(generated, pt)
         lazy val shadowing =
-          typed(untpd.Ident(ref.name) withPos pos.toSynthetic, funProto)(nestedContext.setNewTyperState.addMode(Mode.ImplicitShadowing))
+          typed(untpd.Ident(ref.name) withPos pos.toSynthetic, funProto)
+               (nestedContext.addMode(Mode.ImplicitShadowing).setNewTyperState)
         def refMatches(shadowing: Tree): Boolean =
           ref.symbol == closureBody(shadowing).symbol || {
             shadowing match {